1 files changed, 1358 insertions, 1358 deletions
diff --git a/tex/context/base/spac-ver.lua b/tex/context/base/spac-ver.lua
index a042945f2..7d030ab1a 100644
--- a/tex/context/base/spac-ver.lua
+++ b/tex/context/base/spac-ver.lua
@@ -1,1358 +1,1358 @@
-if not modules then modules = { } end modules ['spac-ver'] = {
-    version   = 1.001,
-    comment   = "companion to spac-ver.mkiv",
-    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
-    copyright = "PRAGMA ADE / ConTeXt Development Team",
-    license   = "see context related readme files"
-}
-
--- we also need to call the spacer for inserts!
-
--- todo: directly set skips
-
--- this code dates from the beginning and is kind of experimental; it
--- will be optimized and improved soon
---
--- the collapser will be redone with user nodes; also, we might get make
--- parskip into an attribute and appy it explicitly thereby getting rid
--- of automated injections; eventually i want to get rid of the currently
--- still needed tex -> lua -> tex > lua chain (needed because we can have
--- expandable settings at the tex end
-
--- todo: strip baselineskip around display math
-
-local next, type, tonumber = next, type, tonumber
-local gmatch, concat = string.gmatch, table.concat
-local ceil, floor, max, min, round, abs = math.ceil, math.floor, math.max, math.min, math.round, math.abs
-local texlists, texdimen, texbox = tex.lists, tex.dimen, tex.box
-local lpegmatch = lpeg.match
-local unpack = unpack or table.unpack
-local allocate = utilities.storage.allocate
-local todimen = string.todimen
-local formatters = string.formatters
-
-local P, C, R, S, Cc = lpeg.P, lpeg.C, lpeg.R, lpeg.S, lpeg.Cc
-
-local nodes, node, trackers, attributes, context =  nodes, node, trackers, attributes, context
-
-local variables   = interfaces.variables
-
-local starttiming = statistics.starttiming
-local stoptiming  = statistics.stoptiming
-
--- vertical space handler
-
-local trace_vbox_vspacing    = false  trackers.register("vspacing.vbox",     function(v) trace_vbox_vspacing    = v end)
-local trace_page_vspacing    = false  trackers.register("vspacing.page",     function(v) trace_page_vspacing    = v end)
-local trace_page_builder     = false  trackers.register("builders.page",     function(v) trace_page_builder     = v end)
-local trace_collect_vspacing = false  trackers.register("vspacing.collect",  function(v) trace_collect_vspacing = v end)
-local trace_vspacing         = false  trackers.register("vspacing.spacing",  function(v) trace_vspacing         = v end)
-local trace_vsnapping        = false  trackers.register("vspacing.snapping", function(v) trace_vsnapping        = v end)
-local trace_vpacking         = false  trackers.register("vspacing.packing",  function(v) trace_vpacking         = v end)
-
-local report_vspacing     = logs.reporter("vspacing","spacing")
-local report_collapser    = logs.reporter("vspacing","collapsing")
-local report_snapper      = logs.reporter("vspacing","snapping")
-local report_page_builder = logs.reporter("builders","page")
-
-local a_skipcategory      = attributes.private('skipcategory')
-local a_skippenalty       = attributes.private('skippenalty')
-local a_skiporder         = attributes.private('skiporder')
------ snap_category       = attributes.private('snapcategory')
-local a_snapmethod        = attributes.private('snapmethod')
-local a_snapvbox          = attributes.private('snapvbox')
-
-local find_node_tail      = node.tail
-local free_node           = node.free
-local free_node_list      = node.flush_list
-local copy_node           = node.copy
-local traverse_nodes      = node.traverse
-local traverse_nodes_id   = node.traverse_id
-local insert_node_before  = node.insert_before
-local insert_node_after   = node.insert_after
-local remove_node         = nodes.remove
-local count_nodes         = nodes.count
-local nodeidstostring     = nodes.idstostring
-local hpack_node          = node.hpack
-local vpack_node          = node.vpack
-local writable_spec       = nodes.writable_spec
-local listtoutf           = nodes.listtoutf
-
-local nodepool            = nodes.pool
-
-local new_penalty         = nodepool.penalty
-local new_kern            = nodepool.kern
-local new_rule            = nodepool.rule
-local new_gluespec        = nodepool.gluespec
-
-local nodecodes           = nodes.nodecodes
-local skipcodes           = nodes.skipcodes
-local fillcodes           = nodes.fillcodes
-
-local penalty_code        = nodecodes.penalty
-local kern_code           = nodecodes.kern
-local glue_code           = nodecodes.glue
-local hlist_code          = nodecodes.hlist
-local vlist_code          = nodecodes.vlist
-local whatsit_code        = nodecodes.whatsit
-
-local userskip_code       = skipcodes.userskip
-
-local vspacing            = builders.vspacing or { }
-builders.vspacing         = vspacing
-
-local vspacingdata        = vspacing.data or { }
-vspacing.data             = vspacingdata
-
-vspacingdata.snapmethods  = vspacingdata.snapmethods or { }
-local snapmethods         = vspacingdata.snapmethods --maybe some older code can go
-
-storage.register("builders/vspacing/data/snapmethods", snapmethods, "builders.vspacing.data.snapmethods")
-
-local default = {
-    maxheight = true,
-    maxdepth  = true,
-    strut     = true,
-    hfraction = 1,
-    dfraction = 1,
-}
-
-local fractions = {
-    minheight = "hfraction", maxheight = "hfraction",
-    mindepth  = "dfraction", maxdepth  = "dfraction",
-    top       = "tlines",    bottom    = "blines",
-}
-
-local values = {
-    offset = "offset"
-}
-
-local colonsplitter = lpeg.splitat(":")
-
-local function listtohash(str)
-    local t = { }
-    for s in gmatch(str,"[^, ]+") do
-        local key, detail = lpegmatch(colonsplitter,s)
-        local v = variables[key]
-        if v then
-            t[v] = true
-            if detail then
-                local k = fractions[key]
-                if k then
-                    detail = tonumber("0" .. detail)
-                    if detail then
-                        t[k] = detail
-                    end
-                else
-                    k = values[key]
-                    if k then
-                        detail = todimen(detail)
-                        if detail then
-                            t[k] = detail
-                        end
-                    end
-                end
-            end
-        else
-            detail = tonumber("0" .. key)
-            if detail then
-                t.hfraction, t.dfraction = detail, detail
-            end
-        end
-    end
-    if next(t) then
-        t.hfraction = t.hfraction or 1
-        t.dfraction = t.dfraction or 1
-        return t
-    else
-        return default
-    end
-end
-
-function vspacing.definesnapmethod(name,method)
-    local n = #snapmethods + 1
-    local t = listtohash(method)
-    snapmethods[n] = t
-    t.name, t.specification = name, method
-    context(n)
-end
-
--- local rule_id  = nodecodes.rule
--- local vlist_id = nodecodes.vlist
--- function nodes.makevtop(n)
---     if n.id == vlist_id then
---         local list = n.list
---         local height = (list and list.id <= rule_id and list.height) or 0
---         n.depth = n.depth - height + n.height
---         n.height = height
---     end
--- end
-
-local reference = nodes.reference
-
-local function validvbox(parentid,list)
-    if parentid == hlist_code then
-        local id = list.id
-        if id == whatsit_code then -- check for initial par subtype
-            list = list.next
-            if not next then
-                return nil
-            end
-        end
-        local done = nil
-        for n in traverse_nodes(list) do
-            local id = n.id
-            if id == vlist_code or id == hlist_code then
-                if done then
-                    return nil
-                else
-                    done = n
-                end
-            elseif id == glue_code or id == penalty_code then
-                -- go on
-            else
-                return nil -- whatever
-            end
-        end
-        if done then
-            local id = done.id
-            if id == hlist_code then
-                return validvbox(id,done.list)
-            end
-        end
-        return done -- only one vbox
-    end
-end
-
-local function already_done(parentid,list,a_snapmethod) -- todo: done when only boxes and all snapped
-    -- problem: any snapped vbox ends up in a line
-    if list and parentid == hlist_code then
-        local id = list.id
-        if id == whatsit_code then -- check for initial par subtype
-            list = list.next
-            if not next then
-                return false
-            end
-        end
---~ local i = 0
-        for n in traverse_nodes(list) do
-            local id = n.id
---~ i = i + 1 print(i,nodecodes[id],n[a_snapmethod])
-            if id == hlist_code or id == vlist_code then
-                local a = n[a_snapmethod]
-                if not a then
-                 -- return true -- not snapped at all
-                elseif a == 0 then
-                    return true -- already snapped
-                end
-            elseif id == glue_code or id == penalty_code then -- whatsit is weak spot
-                -- go on
-            else
-                return false -- whatever
-            end
-        end
-    end
-    return false
-end
-
-
--- quite tricky: ceil(-something) => -0
-
-local function ceiled(n)
-    if n < 0 or n < 0.01 then
-        return 0
-    else
-        return ceil(n)
-    end
-end
-
-local function floored(n)
-    if n < 0 or n < 0.01 then
-        return 0
-    else
-        return floor(n)
-    end
-end
-
--- check variables.none etc
-
-local function snap_hlist(where,current,method,height,depth) -- method.strut is default
-    local list = current.list
-    local t = trace_vsnapping and { }
-    if t then
-        t[#t+1] = formatters["list content: %s"](nodes.toutf(list))
-        t[#t+1] = formatters["parent id: %s"](reference(current))
-        t[#t+1] = formatters["snap method: %s"](method.name)
-        t[#t+1] = formatters["specification: %s"](method.specification)
-    end
-    local snapht, snapdp
-    if method["local"] then
-        -- snapping is done immediately here
-        snapht, snapdp = texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth
-        if t then
-            t[#t+1] = formatters["local: snapht %p snapdp %p"](snapht,snapdp)
-        end
-    elseif method["global"] then
-        snapht, snapdp = texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth
-        if t then
-            t[#t+1] = formatters["global: snapht %p snapdp %p"](snapht,snapdp)
-        end
-    else
-        -- maybe autolocal
-        -- snapping might happen later in the otr
-        snapht, snapdp = texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth
-        local lsnapht, lsnapdp = texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth
-        if snapht ~= lsnapht and snapdp ~= lsnapdp then
-            snapht, snapdp = lsnapht, lsnapdp
-        end
-        if t then
-            t[#t+1] = formatters["auto: snapht %p snapdp %p"](snapht,snapdp)
-        end
-    end
-    local h, d = height or current.height, depth or current.depth
-    local hr, dr, ch, cd = method.hfraction or 1, method.dfraction or 1, h, d
-    local tlines, blines = method.tlines or 1, method.blines or 1
-    local done, plusht, plusdp = false, snapht, snapdp
-    local snaphtdp = snapht + snapdp
-
-    if method.none then
-        plusht, plusdp = 0, 0
-        if t then
-            t[#t+1] = "none: plusht 0pt plusdp 0pt"
-        end
-    end
-    if method.halfline then -- extra halfline
-        plusht, plusdp = plusht + snaphtdp/2, plusdp + snaphtdp/2
-        if t then
-            t[#t+1] = formatters["halfline: plusht %p plusdp %p"](plusht,plusdp)
-        end
-    end
-    if method.line then -- extra line
-        plusht, plusdp = plusht + snaphtdp, plusdp + snaphtdp
-        if t then
-            t[#t+1] = formatters["line: plusht %p plusdp %p"](plusht,plusdp)
-        end
-    end
-
-    if method.first then
-        local thebox = current
-        local id = thebox.id
-        if id == hlist_code then
-            thebox = validvbox(id,thebox.list)
-            id = thebox and thebox.id
-        end
-        if thebox and id == vlist_code then
-            local list, lh, ld = thebox.list
-            for n in traverse_nodes_id(hlist_code,list) do
-                lh, ld = n.height, n.depth
-                break
-            end
-            if lh then
-                local ht, dp = thebox.height, thebox.depth
-                if t then
-                    t[#t+1] = formatters["first line: height %p depth %p"](lh,ld)
-                    t[#t+1] = formatters["dimensions: height %p depth %p"](ht,dp)
-                end
-                local delta = h - lh
-                ch, cd = lh, delta + d
-                h, d = ch, cd
-                local shifted = hpack_node(current.list)
-                shifted.shift = delta
-                current.list = shifted
-                done = true
-                if t then
-                    t[#t+1] = formatters["first: height %p depth %p shift %p"](ch,cd,delta)
-                end
-            elseif t then
-                t[#t+1] = "first: not done, no content"
-            end
-        elseif t then
-            t[#t+1] = "first: not done, no vbox"
-        end
-    elseif method.last then
-        local thebox = current
-        local id = thebox.id
-        if id == hlist_code then
-            thebox = validvbox(id,thebox.list)
-            id = thebox and thebox.id
-        end
-        if thebox and id == vlist_code then
-            local list, lh, ld = thebox.list
-            for n in traverse_nodes_id(hlist_code,list) do
-                lh, ld = n.height, n.depth
-            end
-            if lh then
-                local ht, dp = thebox.height, thebox.depth
-                if t then
-                    t[#t+1] = formatters["last line: height %p depth %p" ](lh,ld)
-                    t[#t+1] = formatters["dimensions: height %p depth %p"](ht,dp)
-                end
-                local delta = d - ld
-                cd, ch = ld, delta + h
-                h, d = ch, cd
-                local shifted = hpack_node(current.list)
-                shifted.shift = delta
-                current.list = shifted
-                done = true
-                if t then
-                    t[#t+1] = formatters["last: height %p depth %p shift %p"](ch,cd,delta)
-                end
-            elseif t then
-                t[#t+1] = "last: not done, no content"
-            end
-        elseif t then
-            t[#t+1] = "last: not done, no vbox"
-        end
-    end
-    if method.minheight then
-        ch = floored((h-hr*snapht)/snaphtdp)*snaphtdp + plusht
-        if t then
-            t[#t+1] = formatters["minheight: %p"](ch)
-        end
-    elseif method.maxheight then
-        ch = ceiled((h-hr*snapht)/snaphtdp)*snaphtdp + plusht
-        if t then
-            t[#t+1] = formatters["maxheight: %p"](ch)
-        end
-    else
-        ch = plusht
-        if t then
-            t[#t+1] = formatters["set height: %p"](ch)
-        end
-    end
-    if method.mindepth then
-        cd = floored((d-dr*snapdp)/snaphtdp)*snaphtdp + plusdp
-        if t then
-            t[#t+1] = formatters["mindepth: %p"](cd)
-        end
-    elseif method.maxdepth then
-        cd = ceiled((d-dr*snapdp)/snaphtdp)*snaphtdp + plusdp
-        if t then
-            t[#t+1] = formatters["maxdepth: %p"](cd)
-        end
-    else
-        cd = plusdp
-        if t then
-            t[#t+1] = formatters["set depth: %p"](cd)
-        end
-    end
-    if method.top then
-        ch = ch + tlines * snaphtdp
-        if t then
-            t[#t+1] = formatters["top height: %p"](ch)
-        end
-    end
-    if method.bottom then
-        cd = cd + blines * snaphtdp
-        if t then
-            t[#t+1] = formatters["bottom depth: %p"](cd)
-        end
-    end
-
-    local offset = method.offset
-    if offset then
-        -- we need to set the attr
-        if t then
-            t[#t+1] = formatters["before offset: %p (width %p height %p depth %p)"](offset,current.width,current.height,current.depth)
-        end
-        local shifted = hpack_node(current.list)
-        shifted.shift = offset
-        current.list = shifted
-        if t then
-            t[#t+1] = formatters["after offset: %p (width %p height %p depth %p)"](offset,current.width,current.height,current.depth)
-        end
-        shifted[a_snapmethod] = 0
-        current[a_snapmethod] = 0
-    end
-    if not height then
-        current.height = ch
-        if t then
-            t[#t+1] = formatters["forced height: %p"](ch)
-        end
-    end
-    if not depth then
-        current.depth = cd
-        if t then
-            t[#t+1] = formatters["forced depth: %p"](cd)
-        end
-    end
-    local lines = (ch+cd)/snaphtdp
-    if t then
-        local original = (h+d)/snaphtdp
-        local whatever = (ch+cd)/(texdimen.globalbodyfontstrutheight + texdimen.globalbodyfontstrutdepth)
-        t[#t+1] = formatters["final lines: %s -> %s (%s)"](original,lines,whatever)
-        t[#t+1] = formatters["final height: %p -> %p"](h,ch)
-        t[#t+1] = formatters["final depth: %p -> %p"](d,cd)
-    end
-    if t then
-        report_snapper("trace: %s type %s\n\t%\n\tt",where,nodecodes[current.id],t)
-    end
-    return h, d, ch, cd, lines
-end
-
-local function snap_topskip(current,method)
-    local spec = current.spec
-    local w = spec.width
-    local wd = w
-    if spec.writable then
-        spec.width, wd = 0, 0
-    end
-    return w, wd
-end
-
-local categories = allocate {
-     [0] = 'discard',
-     [1] = 'largest',
-     [2] = 'force'  ,
-     [3] = 'penalty',
-     [4] = 'add'    ,
-     [5] = 'disable',
-     [6] = 'nowhite',
-     [7] = 'goback',
-     [8] = 'together'
-}
-
-vspacing.categories = categories
-
-function vspacing.tocategories(str)
-    local t = { }
-    for s in gmatch(str,"[^, ]") do
-        local n = tonumber(s)
-        if n then
-            t[categories[n]] = true
-        else
-            t[b] = true
-        end
-    end
-    return t
-end
-
-function vspacing.tocategory(str)
-    if type(str) == "string" then
-        return set.tonumber(vspacing.tocategories(str))
-    else
-        return set.tonumber({ [categories[str]] = true })
-    end
-end
-
-vspacingdata.map  = vspacingdata.map  or { } -- allocate ?
-vspacingdata.skip = vspacingdata.skip or { } -- allocate ?
-
-storage.register("builders/vspacing/data/map",  vspacingdata.map,  "builders.vspacing.data.map")
-storage.register("builders/vspacing/data/skip", vspacingdata.skip, "builders.vspacing.data.skip")
-
-do -- todo: interface.variables
-
-    vspacing.fixed = false
-
-    local map  = vspacingdata.map
-    local skip = vspacingdata.skip
-
-    local multiplier = C(S("+-")^0 * R("09")^1) * P("*")
-    local category   = P(":") * C(P(1)^1)
-    local keyword    = C((1-category)^1)
-    local splitter   = (multiplier + Cc(1)) * keyword * (category + Cc(false))
-
-    local k_fixed, k_flexible, k_category, k_penalty, k_order = variables.fixed, variables.flexible, "category", "penalty", "order"
-
-    -- This will change: just node.write and we can store the values in skips which
-    -- then obeys grouping
-
-    local fixedblankskip         = context.fixedblankskip
-    local flexibleblankskip      = context.flexibleblankskip
-    local setblankcategory       = context.setblankcategory
-    local setblankorder          = context.setblankorder
-    local setblankpenalty        = context.setblankpenalty
-    local setblankhandling       = context.setblankhandling
-    local flushblankhandling     = context.flushblankhandling
-    local addpredefinedblankskip = context.addpredefinedblankskip
-    local addaskedblankskip      = context.addaskedblankskip
-
-    local function analyze(str,oldcategory) -- we could use shorter names
-        for s in gmatch(str,"([^ ,]+)") do
-            local amount, keyword, detail = lpegmatch(splitter,s) -- the comma splitter can be merged
-            if not keyword then
-                report_vspacing("unknown directive %a",s)
-            else
-                local mk = map[keyword]
-                if mk then
-                    category = analyze(mk,category)
-                elseif keyword == k_fixed then
-                    fixedblankskip()
-                elseif keyword == k_flexible then
-                    flexibleblankskip()
-                elseif keyword == k_category then
-                    local category = tonumber(detail)
-                    if category then
-                        setblankcategory(category)
-                        if category ~= oldcategory then
-                            flushblankhandling()
-                            oldcategory = category
-                        end
-                    end
-                elseif keyword == k_order and detail then
-                    local order = tonumber(detail)
-                    if order then
-                        setblankorder(order)
-                    end
-                elseif keyword == k_penalty and detail then
-                    local penalty = tonumber(detail)
-                    if penalty then
-                        setblankpenalty(penalty)
-                    end
-                else
-                    amount = tonumber(amount) or 1
-                    local sk = skip[keyword]
-                    if sk then
-                        addpredefinedblankskip(amount,keyword)
-                    else -- no check
-                        addaskedblankskip(amount,keyword)
-                    end
-                end
-            end
-        end
-        return category
-    end
-
-    local pushlogger         = context.pushlogger
-    local startblankhandling = context.startblankhandling
-    local stopblankhandling  = context.stopblankhandling
-    local poplogger          = context.poplogger
-
-    function vspacing.analyze(str)
-        if trace_vspacing then
-            pushlogger(report_vspacing)
-            startblankhandling()
-            analyze(str,1)
-            stopblankhandling()
-            poplogger()
-        else
-            startblankhandling()
-            analyze(str,1)
-            stopblankhandling()
-        end
-    end
-
-    --
-
-    function vspacing.setmap(from,to)
-        map[from] = to
-    end
-
-    function vspacing.setskip(key,value,grid)
-        if value ~= "" then
-            if grid == "" then grid = value end
-            skip[key] = { value, grid }
-        end
-    end
-
-end
-
--- implementation
-
-local trace_list, tracing_info, before, after = { }, false, "", ""
-
-local function nodes_to_string(head)
-    local current, t = head, { }
-    while current do
-        local id = current.id
-        local ty = nodecodes[id]
-        if id == penalty_code then
-            t[#t+1] = formatters["%s:%s"](ty,current.penalty)
-        elseif id == glue_code then -- or id == kern_code then -- to be tested
-            t[#t+1] = formatters["%s:%p"](ty,current)
-        elseif id == kern_code then
-            t[#t+1] = formatters["%s:%p"](ty,current.kern)
-        else
-            t[#t+1] = ty
-        end
-        current = current.next
-    end
-    return concat(t," + ")
-end
-
-local function reset_tracing(head)
-    trace_list, tracing_info, before, after = { }, false, nodes_to_string(head), ""
-end
-
-local function trace_skip(str,sc,so,sp,data)
-    trace_list[#trace_list+1] = { "skip", formatters["%s | %p | category %s | order %s | penalty %s"](str, data, sc or "-", so or "-", sp or "-") }
-    tracing_info = true
-end
-
-local function trace_natural(str,data)
-    trace_list[#trace_list+1] = { "skip", formatters["%s | %p"](str, data) }
-    tracing_info = true
-end
-
-local function trace_info(message, where, what)
-    trace_list[#trace_list+1] = { "info", formatters["%s: %s/%s"](message,where,what) }
-end
-
-local function trace_node(what)
-    local nt = nodecodes[what.id]
-    local tl = trace_list[#trace_list]
-    if tl and tl[1] == "node" then
-        trace_list[#trace_list] = { "node", formatters["%s + %s"](tl[2],nt) }
-    else
-        trace_list[#trace_list+1] = { "node", nt }
-    end
-end
-
-local function trace_done(str,data)
-    if data.id == penalty_code then
-        trace_list[#trace_list+1] = { "penalty", formatters["%s | %s"](str,data.penalty) }
-    else
-        trace_list[#trace_list+1] = { "glue", formatters["%s | %p"](str,data) }
-    end
-    tracing_info = true
-end
-
-local function show_tracing(head)
-    if tracing_info then
-        after = nodes_to_string(head)
-        for i=1,#trace_list do
-            local tag, text = unpack(trace_list[i])
-            if tag == "info" then
-                report_collapser(text)
-            else
-                report_collapser("  %s: %s",tag,text)
-            end
-        end
-        report_collapser("before: %s",before)
-        report_collapser("after : %s",after)
-    end
-end
-
--- alignment box begin_of_par vmode_par hmode_par insert penalty before_display after_display
-
-local skipcodes = nodes.skipcodes
-
-local userskip_code              = skipcodes.userskip
-local lineskip_code              = skipcodes.lineskip
-local baselineskip_code          = skipcodes.baselineskip
-local parskip_code               = skipcodes.parskip
-local abovedisplayskip_code      = skipcodes.abovedisplayskip
-local belowdisplayskip_code      = skipcodes.belowdisplayskip
-local abovedisplayshortskip_code = skipcodes.abovedisplayshortskip
-local belowdisplayshortskip_code = skipcodes.belowdisplayshortskip
-local topskip_code               = skipcodes.topskip
-local splittopskip_code          = skipcodes.splittopskip
-
-local free_glue_node = free_node
-local discard, largest, force, penalty, add, disable, nowhite, goback, together = 0, 1, 2, 3, 4, 5, 6, 7, 8
-
--- local function free_glue_node(n)
---  -- free_node(n.spec)
---     print("before",n)
---     logs.flush()
---     free_node(n)
---     print("after")
---     logs.flush()
--- end
-
-function vspacing.snapbox(n,how)
-    local sv = snapmethods[how]
-    if sv then
-        local box = texbox[n]
-        local list = box.list
-        if list then
-            local s = list[a_snapmethod]
-            if s == 0 then
-                if trace_vsnapping then
-                --  report_snapper("box list not snapped, already done")
-                end
-            else
-                local ht, dp = box.height, box.depth
-                if false then -- todo: already_done
-                    -- assume that the box is already snapped
-                    if trace_vsnapping then
-                        report_snapper("box list already snapped at (%p,%p): %s",
-                            ht,dp,listtoutf(list))
-                    end
-                else
-                    local h, d, ch, cd, lines = snap_hlist("box",box,sv,ht,dp)
-                    box.height, box.depth = ch, cd
-                    if trace_vsnapping then
-                        report_snapper("box list snapped from (%p,%p) to (%p,%p) using method %a (%s) for %a (%s lines): %s",
-                            h,d,ch,cd,sv.name,sv.specification,"direct",lines,listtoutf(list))
-                    end
-                    box[a_snapmethod] = 0 --
-                    list[a_snapmethod] = 0 -- yes or no
-                end
-            end
-        end
-    end
-end
-
-local function forced_skip(head,current,width,where,trace)
-    if where == "after" then
-        head, current = insert_node_after(head,current,new_rule(0,0,0))
-        head, current = insert_node_after(head,current,new_kern(width))
-        head, current = insert_node_after(head,current,new_rule(0,0,0))
-    else
-        local c = current
-        head, current = insert_node_before(head,current,new_rule(0,0,0))
-        head, current = insert_node_before(head,current,new_kern(width))
-        head, current = insert_node_before(head,current,new_rule(0,0,0))
-        current = c
-    end
-    if trace then
-        report_vspacing("inserting forced skip of %p",width)
-    end
-    return head, current
-end
-
--- penalty only works well when before skip
-
-local function collapser(head,where,what,trace,snap,a_snapmethod) -- maybe also pass tail
-    if trace then
-        reset_tracing(head)
-    end
-    local current, oldhead = head, head
-    local glue_order, glue_data, force_glue = 0, nil, false
-    local penalty_order, penalty_data, natural_penalty = 0, nil, nil
-    local parskip, ignore_parskip, ignore_following, ignore_whitespace, keep_together = nil, false, false, false, false
-    --
-    -- todo: keep_together: between headers
-    --
-    local function flush(why)
-        if penalty_data then
-            local p = new_penalty(penalty_data)
-            if trace then trace_done("flushed due to " .. why,p) end
-            head = insert_node_before(head,current,p)
-        end
-        if glue_data then
-            if force_glue then
-                if trace then trace_done("flushed due to " .. why,glue_data) end
-                head = forced_skip(head,current,glue_data.spec.width,"before",trace)
-                free_glue_node(glue_data)
-            elseif glue_data.spec.writable then
-                if trace then trace_done("flushed due to " .. why,glue_data) end
-                head = insert_node_before(head,current,glue_data)
-            else
-                free_glue_node(glue_data)
-            end
-        end
-        if trace then trace_node(current) end
-        glue_order, glue_data, force_glue = 0, nil, false
-        penalty_order, penalty_data, natural_penalty = 0, nil, nil
-        parskip, ignore_parskip, ignore_following, ignore_whitespace = nil, false, false, false
-    end
-    if trace_vsnapping then
-        report_snapper("global ht/dp = %p/%p, local ht/dp = %p/%p",
-            texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth,
-            texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth)
-    end
-    if trace then trace_info("start analyzing",where,what) end
-    while current do
-        local id = current.id
-        if id == hlist_code or id == vlist_code then
-            -- needs checking, why so many calls
-            if snap then
-                local list = current.list
-                local s = current[a_snapmethod]
-                if not s then
-                --  if trace_vsnapping then
-                --      report_snapper("mvl list not snapped")
-                --  end
-                elseif s == 0 then
-                    if trace_vsnapping then
-                        report_snapper("mvl %a not snapped, already done: %s",nodecodes[id],listtoutf(list))
-                    end
-                else
-                    local sv = snapmethods[s]
-                    if sv then
-                        -- check if already snapped
-                        if list and already_done(id,list,a_snapmethod) then
-                            local ht, dp = current.height, current.depth
-                            -- assume that the box is already snapped
-                            if trace_vsnapping then
-                                report_snapper("mvl list already snapped at (%p,%p): %s",ht,dp,listtoutf(list))
-                            end
-                        else
-                            local h, d, ch, cd, lines = snap_hlist("mvl",current,sv)
-                            if trace_vsnapping then
-                                report_snapper("mvl %a snapped from (%p,%p) to (%p,%p) using method %a (%s) for %a (%s lines): %s",
-                                    nodecodes[id],h,d,ch,cd,sv.name,sv.specification,where,lines,listtoutf(list))
-                            end
-                        end
-                    elseif trace_vsnapping then
-                        report_snapper("mvl %a not snapped due to unknown snap specification: %s",nodecodes[id],listtoutf(list))
-                    end
-                    current[a_snapmethod] = 0
-                end
-            else
-                --
-            end
-        --  tex.prevdepth = 0
-            flush("list")
-            current = current.next
-        elseif id == penalty_code then
-         -- natural_penalty = current.penalty
-         -- if trace then trace_done("removed penalty",current) end
-         -- head, current = remove_node(head, current, true)
-            current = current.next
-        elseif id == kern_code then
-            if snap and trace_vsnapping and current.kern ~= 0 then
-                report_snapper("kern of %p kept",current.kern)
-            end
-            flush("kern")
-            current = current.next
-        elseif id == glue_code then
-            local subtype = current.subtype
-            if subtype == userskip_code then
-                local sc = current[a_skipcategory]   -- has no default, no unset (yet)
-                local so = current[a_skiporder] or 1 -- has  1 default, no unset (yet)
-                local sp = current[a_skippenalty]    -- has no default, no unset (yet)
-                if sp and sc == penalty then
-                    if not penalty_data then
-                        penalty_data = sp
-                    elseif penalty_order < so then
-                        penalty_order, penalty_data = so, sp
-                    elseif penalty_order == so and sp > penalty_data then
-                        penalty_data = sp
-                    end
-                    if trace then trace_skip("penalty in skip",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                elseif not sc then  -- if not sc then
-                    if glue_data then
-                        if trace then trace_done("flush",glue_data) end
-                        head = insert_node_before(head,current,glue_data)
-                        if trace then trace_natural("natural",current) end
-                        current = current.next
-                    else
-                        -- not look back across head
-                        local previous = current.prev
-                        if previous and previous.id == glue_code and previous.subtype == userskip_code then
-                            local ps = previous.spec
-                            if ps.writable then
-                                local cs = current.spec
-                                if cs.writable and ps.stretch_order == 0 and ps.shrink_order == 0 and cs.stretch_order == 0 and cs.shrink_order == 0 then
-                                    local pw, pp, pm = ps.width, ps.stretch, ps.shrink
-                                    local cw, cp, cm = cs.width, cs.stretch, cs.shrink
-                                 -- ps = writable_spec(previous) -- no writable needed here
-                                 -- ps.width, ps.stretch, ps.shrink = pw + cw, pp + cp, pm + cm
-                                    previous.spec = new_gluespec(pw + cw, pp + cp, pm + cm) -- else topskip can disappear
-                                    if trace then trace_natural("removed",current) end
-                                    head, current = remove_node(head, current, true)
-                                --  current = previous
-                                    if trace then trace_natural("collapsed",previous) end
-                                --  current = current.next
-                                else
-                                    if trace then trace_natural("filler",current) end
-                                    current = current.next
-                                end
-                            else
-                                if trace then trace_natural("natural (no prev spec)",current) end
-                                current = current.next
-                            end
-                        else
-                            if trace then trace_natural("natural (no prev)",current) end
-                            current = current.next
-                        end
-                    end
-                    glue_order, glue_data = 0, nil
-                elseif sc == disable then
-                    ignore_following = true
-                    if trace then trace_skip("disable",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                elseif sc == together then
-                    keep_together = true
-                    if trace then trace_skip("together",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                elseif sc == nowhite then
-                    ignore_whitespace = true
-                    head, current = remove_node(head, current, true)
-                elseif sc == discard then
-                    if trace then trace_skip("discard",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                elseif ignore_following then
-                    if trace then trace_skip("disabled",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                elseif not glue_data then
-                    if trace then trace_skip("assign",sc,so,sp,current) end
-                    glue_order = so
-                    head, current, glue_data = remove_node(head, current)
-                elseif glue_order < so then
-                    if trace then trace_skip("force",sc,so,sp,current) end
-                    glue_order = so
-                    free_glue_node(glue_data)
-                    head, current, glue_data = remove_node(head, current)
-                elseif glue_order == so then
-                    -- is now exclusive, maybe support goback as combi, else why a set
-                    if sc == largest then
-                        local cs, gs = current.spec, glue_data.spec
-                        local cw, gw = cs.width, gs.width
-                        if cw > gw then
-                            if trace then trace_skip("largest",sc,so,sp,current) end
-                            free_glue_node(glue_data) -- also free spec
-                            head, current, glue_data = remove_node(head, current)
-                        else
-                            if trace then trace_skip("remove smallest",sc,so,sp,current) end
-                            head, current = remove_node(head, current, true)
-                        end
-                    elseif sc == goback then
-                        if trace then trace_skip("goback",sc,so,sp,current) end
-                        free_glue_node(glue_data) -- also free spec
-                        head, current, glue_data = remove_node(head, current)
-                    elseif sc == force then
-                        -- last one counts, some day we can provide an accumulator and largest etc
-                        -- but not now
-                        if trace then trace_skip("force",sc,so,sp,current) end
-                        free_glue_node(glue_data) -- also free spec
-                        head, current, glue_data = remove_node(head, current)
-                    elseif sc == penalty then
-                        if trace then trace_skip("penalty",sc,so,sp,current) end
-                        free_glue_node(glue_data) -- also free spec
-                        glue_data = nil
-                        head, current = remove_node(head, current, true)
-                    elseif sc == add then
-                        if trace then trace_skip("add",sc,so,sp,current) end
-                     -- local old, new = glue_data.spec, current.spec
-                        local old, new = writable_spec(glue_data), current.spec
-                        old.width   = old.width   + new.width
-                        old.stretch = old.stretch + new.stretch
-                        old.shrink  = old.shrink  + new.shrink
-                        -- toto: order
-                        head, current = remove_node(head, current, true)
-                    else
-                        if trace then trace_skip("unknown",sc,so,sp,current) end
-                        head, current = remove_node(head, current, true)
-                    end
-                else
-                    if trace then trace_skip("unknown",sc,so,sp,current) end
-                    head, current = remove_node(head, current, true)
-                end
-                if sc == force then
-                    force_glue = true
-                end
-            elseif subtype == lineskip_code then
-                if snap then
-                    local s = current[a_snapmethod]
-                    if s and s ~= 0 then
-                        current[a_snapmethod] = 0
-                        if current.spec.writable then
-                            local spec = writable_spec(current)
-                            spec.width = 0
-                            if trace_vsnapping then
-                                report_snapper("lineskip set to zero")
-                            end
-                        end
-                    else
-                        if trace then trace_skip("lineskip",sc,so,sp,current) end
-                        flush("lineskip")
-                    end
-                else
-                    if trace then trace_skip("lineskip",sc,so,sp,current) end
-                    flush("lineskip")
-                end
-                current = current.next
-            elseif subtype == baselineskip_code then
-                if snap then
-                    local s = current[a_snapmethod]
-                    if s and s ~= 0 then
-                        current[a_snapmethod] = 0
-                        if current.spec.writable then
-                            local spec = writable_spec(current)
-                            spec.width = 0
-                            if trace_vsnapping then
-                                report_snapper("baselineskip set to zero")
-                            end
-                        end
-                    else
-                        if trace then trace_skip("baselineskip",sc,so,sp,current) end
-                        flush("baselineskip")
-                    end
-                else
-                    if trace then trace_skip("baselineskip",sc,so,sp,current) end
-                    flush("baselineskip")
-                end
-                current = current.next
-            elseif subtype == parskip_code then
-                -- parskip always comes later
-                if ignore_whitespace then
-                    if trace then trace_natural("ignored parskip",current) end
-                    head, current = remove_node(head, current, true)
-                elseif glue_data then
-                    local ps, gs = current.spec, glue_data.spec
-                    if ps.writable and gs.writable and ps.width > gs.width then
-                        glue_data.spec = copy_node(ps)
-                        if trace then trace_natural("taking parskip",current) end
-                    else
-                        if trace then trace_natural("removed parskip",current) end
-                    end
-                    head, current = remove_node(head, current, true)
-                else
-                    if trace then trace_natural("honored parskip",current) end
-                    head, current, glue_data = remove_node(head, current)
-                end
-            elseif subtype == topskip_code or subtype == splittopskip_code then
-                if snap then
-                    local s = current[a_snapmethod]
-                    if s and s ~= 0 then
-                        current[a_snapmethod] = 0
-                        local sv = snapmethods[s]
-                        local w, cw = snap_topskip(current,sv)
-                        if trace_vsnapping then
-                            report_snapper("topskip snapped from %p to %p for %a",w,cw,where)
-                        end
-                    else
-                        if trace then trace_skip("topskip",sc,so,sp,current) end
-                        flush("topskip")
-                    end
-                else
-                    if trace then trace_skip("topskip",sc,so,sp,current) end
-                    flush("topskip")
-                end
-                current = current.next
-            elseif subtype == abovedisplayskip_code then
-                --
-                if trace then trace_skip("above display skip (normal)",sc,so,sp,current) end
-                flush("above display skip (normal)")
-                current = current.next
-                --
-            elseif subtype == belowdisplayskip_code then
-                --
-                if trace then trace_skip("below display skip (normal)",sc,so,sp,current) end
-                flush("below display skip (normal)")
-                current = current.next
-                --
-            elseif subtype == abovedisplayshortskip_code then
-                --
-                if trace then trace_skip("above display skip (short)",sc,so,sp,current) end
-                flush("above display skip (short)")
-                current = current.next
-                --
-            elseif subtype == belowdisplayshortskip_code then
-                --
-                if trace then trace_skip("below display skip (short)",sc,so,sp,current) end
-                flush("below display skip (short)")
-                current = current.next
-                --
-            else -- other glue
-                if snap and trace_vsnapping and current.spec.writable and current.spec.width ~= 0 then
-                    report_snapper("glue %p of type %a kept",current.spec.width,skipcodes[subtype])
-                --~ current.spec.width = 0
-                end
-                if trace then trace_skip(formatted["glue of type %a"](subtype),sc,so,sp,current) end
-                flush("some glue")
-                current = current.next
-            end
-        else
-            flush("something else")
-            current = current.next
-        end
-    end
-    if trace then trace_info("stop analyzing",where,what) end
- -- if natural_penalty and (not penalty_data or natural_penalty > penalty_data) then
- --     penalty_data = natural_penalty
- -- end
-    if trace and (glue_data or penalty_data) then
-        trace_info("start flushing",where,what)
-    end
-    local tail
-    if penalty_data then
-        tail = find_node_tail(head)
-        local p = new_penalty(penalty_data)
-        if trace then trace_done("result",p) end
-        head, tail = insert_node_after(head,tail,p)
-    end
-    if glue_data then
-        if not tail then tail = find_node_tail(head) end
-        if trace then trace_done("result",glue_data) end
-        if force_glue then
-            head, tail = forced_skip(head,tail,glue_data.spec.width,"after",trace)
-            free_glue_node(glue_data)
-        else
-            head, tail = insert_node_after(head,tail,glue_data)
-        end
-    end
-    if trace then
-        if glue_data or penalty_data then
-            trace_info("stop flushing",where,what)
-        end
-        show_tracing(head)
-        if oldhead ~= head then
-            trace_info("head has been changed from %a to %a",nodecodes[oldhead.id],nodecodes[head.id])
-        end
-    end
-    return head, true
-end
-
--- alignment after_output end box new_graf vmode_par hmode_par insert penalty before_display after_display
--- \par -> vmode_par
---
--- status.best_page_break
--- tex.lists.best_page_break
--- tex.lists.best_size (natural size to best_page_break)
--- tex.lists.least_page_cost (badness of best_page_break)
--- tex.lists.page_head
--- tex.lists.contrib_head
-
-local stackhead, stacktail, stackhack = nil, nil, false
-
-local function report(message,lst)
-    report_vspacing(message,count_nodes(lst,true),nodeidstostring(lst))
-end
-
-function vspacing.pagehandler(newhead,where)
-    -- local newhead = texlists.contrib_head
-    if newhead then
-        local newtail = find_node_tail(newhead) -- best pass that tail, known anyway
-        local flush = false
-        stackhack = true -- todo: only when grid snapping once enabled
-        for n in traverse_nodes(newhead) do -- we could just look for glue nodes
-            local id = n.id
-            if id ~= glue_code then
-                flush = true
-            elseif n.subtype == userskip_code then
-                if n[a_skipcategory] then
-                    stackhack = true
-                else
-                    flush = true
-                end
-            else
-                -- tricky
-            end
-        end
-        if flush then
-            if stackhead then
-                if trace_collect_vspacing then report("appending %s nodes to stack (final): %s",newhead) end
-                stacktail.next = newhead
-                newhead.prev = stacktail
-                newhead = stackhead
-                stackhead, stacktail = nil, nil
-            end
-            if stackhack then
-                stackhack = false
-                if trace_collect_vspacing then report("processing %s nodes: %s",newhead) end
---~                 texlists.contrib_head = collapser(newhead,"page",where,trace_page_vspacing,true,a_snapmethod)
-                    newhead = collapser(newhead,"page",where,trace_page_vspacing,true,a_snapmethod)
-            else
-                if trace_collect_vspacing then report("flushing %s nodes: %s",newhead) end
---~                 texlists.contrib_head = newhead
-            end
-        else
-            if stackhead then
-                if trace_collect_vspacing then report("appending %s nodes to stack (intermediate): %s",newhead) end
-                stacktail.next = newhead
-                newhead.prev = stacktail
-            else
-                if trace_collect_vspacing then report("storing %s nodes in stack (initial): %s",newhead) end
-                stackhead = newhead
-            end
-            stacktail = newtail
-         -- texlists.contrib_head = nil
-            newhead = nil
-        end
-    end
-    return newhead
-end
-
-local ignore = table.tohash {
-    "split_keep",
-    "split_off",
- -- "vbox",
-}
-
-function vspacing.vboxhandler(head,where)
-    if head and not ignore[where] and head.next then
-    --  starttiming(vspacing)
-        head = collapser(head,"vbox",where,trace_vbox_vspacing,true,a_snapvbox) -- todo: local snapper
-    --  stoptiming(vspacing)
-    end
-    return head
-end
-
-function vspacing.collapsevbox(n) -- for boxes but using global a_snapmethod
-    local list = texbox[n].list
-    if list then
-    --  starttiming(vspacing)
-        texbox[n].list = vpack_node(collapser(list,"snapper","vbox",trace_vbox_vspacing,true,a_snapmethod))
-    --  stoptiming(vspacing)
-    end
-end
-
--- We will split this module so a few locals are repeated. Also this will be
--- rewritten.
-
-nodes.builders = nodes.builder or { }
-local builders = nodes.builders
-
-local actions = nodes.tasks.actions("vboxbuilders")
-
-function builders.vpack_filter(head,groupcode,size,packtype,maxdepth,direction)
-    local done = false
-    if head then
-        starttiming(builders)
-        if trace_vpacking then
-            local before = nodes.count(head)
-            head, done = actions(head,groupcode,size,packtype,maxdepth,direction)
-            local after = nodes.count(head)
-            if done then
-                nodes.processors.tracer("vpack","changed",head,groupcode,before,after,true)
-            else
-                nodes.processors.tracer("vpack","unchanged",head,groupcode,before,after,true)
-            end
-        else
-            head, done = actions(head,groupcode)
-        end
-        stoptiming(builders)
-    end
-    return head, done
-end
-
--- This one is special in the sense that it has no head and we operate on the mlv. Also,
--- we need to do the vspacing last as it removes items from the mvl.
-
-local actions = nodes.tasks.actions("mvlbuilders")
-
-local function report(groupcode,head)
-    report_page_builder("trigger: %s",groupcode)
-    report_page_builder("  vsize    : %p",tex.vsize)
-    report_page_builder("  pagegoal : %p",tex.pagegoal)
-    report_page_builder("  pagetotal: %p",tex.pagetotal)
-    report_page_builder("  list     : %s",head and nodeidstostring(head) or "<empty>")
-end
-
-function builders.buildpage_filter(groupcode)
-    local head, done = texlists.contrib_head, false
- -- if head and head.next and head.next.id == hlist_code and head.next.width == 1 then
- --     report_page_builder("trigger otr calculations")
- --     free_node_list(head)
- --     head = nil
- -- end
-    if head then
-        starttiming(builders)
-        if trace_page_builder then
-            report(groupcode,head)
-        end
-        head, done = actions(head,groupcode)
-        stoptiming(builders)
-     -- -- doesn't work here (not passed on?)
-     -- tex.pagegoal = tex.vsize - tex.dimen.d_page_floats_inserted_top - tex.dimen.d_page_floats_inserted_bottom
-        texlists.contrib_head = head
-        return done and head or true
-    else
-        if trace_page_builder then
-            report(groupcode)
-        end
-        return nil, false
-    end
-end
-
-callbacks.register('vpack_filter',     builders.vpack_filter,     "vertical spacing etc")
-callbacks.register('buildpage_filter', builders.buildpage_filter, "vertical spacing etc (mvl)")
-
-statistics.register("v-node processing time", function()
-    return statistics.elapsedseconds(builders)
-end)
-
--- interface
-
-commands.vspacing          = vspacing.analyze
-commands.vspacingsetamount = vspacing.setskip
-commands.vspacingdefine    = vspacing.setmap
-commands.vspacingcollapse  = vspacing.collapsevbox
-commands.vspacingsnap      = vspacing.snapbox
+if not modules then modules = { } end modules ['spac-ver'] = {
+    version   = 1.001,
+    comment   = "companion to spac-ver.mkiv",
+    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
+    copyright = "PRAGMA ADE / ConTeXt Development Team",
+    license   = "see context related readme files"
+}
+
+-- we also need to call the spacer for inserts!
+
+-- todo: directly set skips
+
+-- this code dates from the beginning and is kind of experimental; it
+-- will be optimized and improved soon
+--
+-- the collapser will be redone with user nodes; also, we might get make
+-- parskip into an attribute and appy it explicitly thereby getting rid
+-- of automated injections; eventually i want to get rid of the currently
+-- still needed tex -> lua -> tex > lua chain (needed because we can have
+-- expandable settings at the tex end
+
+-- todo: strip baselineskip around display math
+
+local next, type, tonumber = next, type, tonumber
+local gmatch, concat = string.gmatch, table.concat
+local ceil, floor, max, min, round, abs = math.ceil, math.floor, math.max, math.min, math.round, math.abs
+local texlists, texdimen, texbox = tex.lists, tex.dimen, tex.box
+local lpegmatch = lpeg.match
+local unpack = unpack or table.unpack
+local allocate = utilities.storage.allocate
+local todimen = string.todimen
+local formatters = string.formatters
+
+local P, C, R, S, Cc = lpeg.P, lpeg.C, lpeg.R, lpeg.S, lpeg.Cc
+
+local nodes, node, trackers, attributes, context =  nodes, node, trackers, attributes, context
+
+local variables   = interfaces.variables
+
+local starttiming = statistics.starttiming
+local stoptiming  = statistics.stoptiming
+
+-- vertical space handler
+
+local trace_vbox_vspacing    = false  trackers.register("vspacing.vbox",     function(v) trace_vbox_vspacing    = v end)
+local trace_page_vspacing    = false  trackers.register("vspacing.page",     function(v) trace_page_vspacing    = v end)
+local trace_page_builder     = false  trackers.register("builders.page",     function(v) trace_page_builder     = v end)
+local trace_collect_vspacing = false  trackers.register("vspacing.collect",  function(v) trace_collect_vspacing = v end)
+local trace_vspacing         = false  trackers.register("vspacing.spacing",  function(v) trace_vspacing         = v end)
+local trace_vsnapping        = false  trackers.register("vspacing.snapping", function(v) trace_vsnapping        = v end)
+local trace_vpacking         = false  trackers.register("vspacing.packing",  function(v) trace_vpacking         = v end)
+
+local report_vspacing     = logs.reporter("vspacing","spacing")
+local report_collapser    = logs.reporter("vspacing","collapsing")
+local report_snapper      = logs.reporter("vspacing","snapping")
+local report_page_builder = logs.reporter("builders","page")
+
+local a_skipcategory      = attributes.private('skipcategory')
+local a_skippenalty       = attributes.private('skippenalty')
+local a_skiporder         = attributes.private('skiporder')
+----- snap_category       = attributes.private('snapcategory')
+local a_snapmethod        = attributes.private('snapmethod')
+local a_snapvbox          = attributes.private('snapvbox')
+
+local find_node_tail      = node.tail
+local free_node           = node.free
+local free_node_list      = node.flush_list
+local copy_node           = node.copy
+local traverse_nodes      = node.traverse
+local traverse_nodes_id   = node.traverse_id
+local insert_node_before  = node.insert_before
+local insert_node_after   = node.insert_after
+local remove_node         = nodes.remove
+local count_nodes         = nodes.count
+local nodeidstostring     = nodes.idstostring
+local hpack_node          = node.hpack
+local vpack_node          = node.vpack
+local writable_spec       = nodes.writable_spec
+local listtoutf           = nodes.listtoutf
+
+local nodepool            = nodes.pool
+
+local new_penalty         = nodepool.penalty
+local new_kern            = nodepool.kern
+local new_rule            = nodepool.rule
+local new_gluespec        = nodepool.gluespec
+
+local nodecodes           = nodes.nodecodes
+local skipcodes           = nodes.skipcodes
+local fillcodes           = nodes.fillcodes
+
+local penalty_code        = nodecodes.penalty
+local kern_code           = nodecodes.kern
+local glue_code           = nodecodes.glue
+local hlist_code          = nodecodes.hlist
+local vlist_code          = nodecodes.vlist
+local whatsit_code        = nodecodes.whatsit
+
+local userskip_code       = skipcodes.userskip
+
+local vspacing            = builders.vspacing or { }
+builders.vspacing         = vspacing
+
+local vspacingdata        = vspacing.data or { }
+vspacing.data             = vspacingdata
+
+vspacingdata.snapmethods  = vspacingdata.snapmethods or { }
+local snapmethods         = vspacingdata.snapmethods --maybe some older code can go
+
+storage.register("builders/vspacing/data/snapmethods", snapmethods, "builders.vspacing.data.snapmethods")
+
+local default = {
+    maxheight = true,
+    maxdepth  = true,
+    strut     = true,
+    hfraction = 1,
+    dfraction = 1,
+}
+
+local fractions = {
+    minheight = "hfraction", maxheight = "hfraction",
+    mindepth  = "dfraction", maxdepth  = "dfraction",
+    top       = "tlines",    bottom    = "blines",
+}
+
+local values = {
+    offset = "offset"
+}
+
+local colonsplitter = lpeg.splitat(":")
+
+local function listtohash(str)
+    local t = { }
+    for s in gmatch(str,"[^, ]+") do
+        local key, detail = lpegmatch(colonsplitter,s)
+        local v = variables[key]
+        if v then
+            t[v] = true
+            if detail then
+                local k = fractions[key]
+                if k then
+                    detail = tonumber("0" .. detail)
+                    if detail then
+                        t[k] = detail
+                    end
+                else
+                    k = values[key]
+                    if k then
+                        detail = todimen(detail)
+                        if detail then
+                            t[k] = detail
+                        end
+                    end
+                end
+            end
+        else
+            detail = tonumber("0" .. key)
+            if detail then
+                t.hfraction, t.dfraction = detail, detail
+            end
+        end
+    end
+    if next(t) then
+        t.hfraction = t.hfraction or 1
+        t.dfraction = t.dfraction or 1
+        return t
+    else
+        return default
+    end
+end
+
+function vspacing.definesnapmethod(name,method)
+    local n = #snapmethods + 1
+    local t = listtohash(method)
+    snapmethods[n] = t
+    t.name, t.specification = name, method
+    context(n)
+end
+
+-- local rule_id  = nodecodes.rule
+-- local vlist_id = nodecodes.vlist
+-- function nodes.makevtop(n)
+--     if n.id == vlist_id then
+--         local list = n.list
+--         local height = (list and list.id <= rule_id and list.height) or 0
+--         n.depth = n.depth - height + n.height
+--         n.height = height
+--     end
+-- end
+
+local reference = nodes.reference
+
+local function validvbox(parentid,list)
+    if parentid == hlist_code then
+        local id = list.id
+        if id == whatsit_code then -- check for initial par subtype
+            list = list.next
+            if not next then
+                return nil
+            end
+        end
+        local done = nil
+        for n in traverse_nodes(list) do
+            local id = n.id
+            if id == vlist_code or id == hlist_code then
+                if done then
+                    return nil
+                else
+                    done = n
+                end
+            elseif id == glue_code or id == penalty_code then
+                -- go on
+            else
+                return nil -- whatever
+            end
+        end
+        if done then
+            local id = done.id
+            if id == hlist_code then
+                return validvbox(id,done.list)
+            end
+        end
+        return done -- only one vbox
+    end
+end
+
+local function already_done(parentid,list,a_snapmethod) -- todo: done when only boxes and all snapped
+    -- problem: any snapped vbox ends up in a line
+    if list and parentid == hlist_code then
+        local id = list.id
+        if id == whatsit_code then -- check for initial par subtype
+            list = list.next
+            if not next then
+                return false
+            end
+        end
+--~ local i = 0
+        for n in traverse_nodes(list) do
+            local id = n.id
+--~ i = i + 1 print(i,nodecodes[id],n[a_snapmethod])
+            if id == hlist_code or id == vlist_code then
+                local a = n[a_snapmethod]
+                if not a then
+                 -- return true -- not snapped at all
+                elseif a == 0 then
+                    return true -- already snapped
+                end
+            elseif id == glue_code or id == penalty_code then -- whatsit is weak spot
+                -- go on
+            else
+                return false -- whatever
+            end
+        end
+    end
+    return false
+end
+
+
+-- quite tricky: ceil(-something) => -0
+
+local function ceiled(n)
+    if n < 0 or n < 0.01 then
+        return 0
+    else
+        return ceil(n)
+    end
+end
+
+local function floored(n)
+    if n < 0 or n < 0.01 then
+        return 0
+    else
+        return floor(n)
+    end
+end
+
+-- check variables.none etc
+
+local function snap_hlist(where,current,method,height,depth) -- method.strut is default
+    local list = current.list
+    local t = trace_vsnapping and { }
+    if t then
+        t[#t+1] = formatters["list content: %s"](nodes.toutf(list))
+        t[#t+1] = formatters["parent id: %s"](reference(current))
+        t[#t+1] = formatters["snap method: %s"](method.name)
+        t[#t+1] = formatters["specification: %s"](method.specification)
+    end
+    local snapht, snapdp
+    if method["local"] then
+        -- snapping is done immediately here
+        snapht, snapdp = texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth
+        if t then
+            t[#t+1] = formatters["local: snapht %p snapdp %p"](snapht,snapdp)
+        end
+    elseif method["global"] then
+        snapht, snapdp = texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth
+        if t then
+            t[#t+1] = formatters["global: snapht %p snapdp %p"](snapht,snapdp)
+        end
+    else
+        -- maybe autolocal
+        -- snapping might happen later in the otr
+        snapht, snapdp = texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth
+        local lsnapht, lsnapdp = texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth
+        if snapht ~= lsnapht and snapdp ~= lsnapdp then
+            snapht, snapdp = lsnapht, lsnapdp
+        end
+        if t then
+            t[#t+1] = formatters["auto: snapht %p snapdp %p"](snapht,snapdp)
+        end
+    end
+    local h, d = height or current.height, depth or current.depth
+    local hr, dr, ch, cd = method.hfraction or 1, method.dfraction or 1, h, d
+    local tlines, blines = method.tlines or 1, method.blines or 1
+    local done, plusht, plusdp = false, snapht, snapdp
+    local snaphtdp = snapht + snapdp
+
+    if method.none then
+        plusht, plusdp = 0, 0
+        if t then
+            t[#t+1] = "none: plusht 0pt plusdp 0pt"
+        end
+    end
+    if method.halfline then -- extra halfline
+        plusht, plusdp = plusht + snaphtdp/2, plusdp + snaphtdp/2
+        if t then
+            t[#t+1] = formatters["halfline: plusht %p plusdp %p"](plusht,plusdp)
+        end
+    end
+    if method.line then -- extra line
+        plusht, plusdp = plusht + snaphtdp, plusdp + snaphtdp
+        if t then
+            t[#t+1] = formatters["line: plusht %p plusdp %p"](plusht,plusdp)
+        end
+    end
+
+    if method.first then
+        local thebox = current
+        local id = thebox.id
+        if id == hlist_code then
+            thebox = validvbox(id,thebox.list)
+            id = thebox and thebox.id
+        end
+        if thebox and id == vlist_code then
+            local list, lh, ld = thebox.list
+            for n in traverse_nodes_id(hlist_code,list) do
+                lh, ld = n.height, n.depth
+                break
+            end
+            if lh then
+                local ht, dp = thebox.height, thebox.depth
+                if t then
+                    t[#t+1] = formatters["first line: height %p depth %p"](lh,ld)
+                    t[#t+1] = formatters["dimensions: height %p depth %p"](ht,dp)
+                end
+                local delta = h - lh
+                ch, cd = lh, delta + d
+                h, d = ch, cd
+                local shifted = hpack_node(current.list)
+                shifted.shift = delta
+                current.list = shifted
+                done = true
+                if t then
+                    t[#t+1] = formatters["first: height %p depth %p shift %p"](ch,cd,delta)
+                end
+            elseif t then
+                t[#t+1] = "first: not done, no content"
+            end
+        elseif t then
+            t[#t+1] = "first: not done, no vbox"
+        end
+    elseif method.last then
+        local thebox = current
+        local id = thebox.id
+        if id == hlist_code then
+            thebox = validvbox(id,thebox.list)
+            id = thebox and thebox.id
+        end
+        if thebox and id == vlist_code then
+            local list, lh, ld = thebox.list
+            for n in traverse_nodes_id(hlist_code,list) do
+                lh, ld = n.height, n.depth
+            end
+            if lh then
+                local ht, dp = thebox.height, thebox.depth
+                if t then
+                    t[#t+1] = formatters["last line: height %p depth %p" ](lh,ld)
+                    t[#t+1] = formatters["dimensions: height %p depth %p"](ht,dp)
+                end
+                local delta = d - ld
+                cd, ch = ld, delta + h
+                h, d = ch, cd
+                local shifted = hpack_node(current.list)
+                shifted.shift = delta
+                current.list = shifted
+                done = true
+                if t then
+                    t[#t+1] = formatters["last: height %p depth %p shift %p"](ch,cd,delta)
+                end
+            elseif t then
+                t[#t+1] = "last: not done, no content"
+            end
+        elseif t then
+            t[#t+1] = "last: not done, no vbox"
+        end
+    end
+    if method.minheight then
+        ch = floored((h-hr*snapht)/snaphtdp)*snaphtdp + plusht
+        if t then
+            t[#t+1] = formatters["minheight: %p"](ch)
+        end
+    elseif method.maxheight then
+        ch = ceiled((h-hr*snapht)/snaphtdp)*snaphtdp + plusht
+        if t then
+            t[#t+1] = formatters["maxheight: %p"](ch)
+        end
+    else
+        ch = plusht
+        if t then
+            t[#t+1] = formatters["set height: %p"](ch)
+        end
+    end
+    if method.mindepth then
+        cd = floored((d-dr*snapdp)/snaphtdp)*snaphtdp + plusdp
+        if t then
+            t[#t+1] = formatters["mindepth: %p"](cd)
+        end
+    elseif method.maxdepth then
+        cd = ceiled((d-dr*snapdp)/snaphtdp)*snaphtdp + plusdp
+        if t then
+            t[#t+1] = formatters["maxdepth: %p"](cd)
+        end
+    else
+        cd = plusdp
+        if t then
+            t[#t+1] = formatters["set depth: %p"](cd)
+        end
+    end
+    if method.top then
+        ch = ch + tlines * snaphtdp
+        if t then
+            t[#t+1] = formatters["top height: %p"](ch)
+        end
+    end
+    if method.bottom then
+        cd = cd + blines * snaphtdp
+        if t then
+            t[#t+1] = formatters["bottom depth: %p"](cd)
+        end
+    end
+
+    local offset = method.offset
+    if offset then
+        -- we need to set the attr
+        if t then
+            t[#t+1] = formatters["before offset: %p (width %p height %p depth %p)"](offset,current.width,current.height,current.depth)
+        end
+        local shifted = hpack_node(current.list)
+        shifted.shift = offset
+        current.list = shifted
+        if t then
+            t[#t+1] = formatters["after offset: %p (width %p height %p depth %p)"](offset,current.width,current.height,current.depth)
+        end
+        shifted[a_snapmethod] = 0
+        current[a_snapmethod] = 0
+    end
+    if not height then
+        current.height = ch
+        if t then
+            t[#t+1] = formatters["forced height: %p"](ch)
+        end
+    end
+    if not depth then
+        current.depth = cd
+        if t then
+            t[#t+1] = formatters["forced depth: %p"](cd)
+        end
+    end
+    local lines = (ch+cd)/snaphtdp
+    if t then
+        local original = (h+d)/snaphtdp
+        local whatever = (ch+cd)/(texdimen.globalbodyfontstrutheight + texdimen.globalbodyfontstrutdepth)
+        t[#t+1] = formatters["final lines: %s -> %s (%s)"](original,lines,whatever)
+        t[#t+1] = formatters["final height: %p -> %p"](h,ch)
+        t[#t+1] = formatters["final depth: %p -> %p"](d,cd)
+    end
+    if t then
+        report_snapper("trace: %s type %s\n\t%\n\tt",where,nodecodes[current.id],t)
+    end
+    return h, d, ch, cd, lines
+end
+
+local function snap_topskip(current,method)
+    local spec = current.spec
+    local w = spec.width
+    local wd = w
+    if spec.writable then
+        spec.width, wd = 0, 0
+    end
+    return w, wd
+end
+
+local categories = allocate {
+     [0] = 'discard',
+     [1] = 'largest',
+     [2] = 'force'  ,
+     [3] = 'penalty',
+     [4] = 'add'    ,
+     [5] = 'disable',
+     [6] = 'nowhite',
+     [7] = 'goback',
+     [8] = 'together'
+}
+
+vspacing.categories = categories
+
+function vspacing.tocategories(str)
+    local t = { }
+    for s in gmatch(str,"[^, ]") do
+        local n = tonumber(s)
+        if n then
+            t[categories[n]] = true
+        else
+            t[b] = true
+        end
+    end
+    return t
+end
+
+function vspacing.tocategory(str)
+    if type(str) == "string" then
+        return set.tonumber(vspacing.tocategories(str))
+    else
+        return set.tonumber({ [categories[str]] = true })
+    end
+end
+
+vspacingdata.map  = vspacingdata.map  or { } -- allocate ?
+vspacingdata.skip = vspacingdata.skip or { } -- allocate ?
+
+storage.register("builders/vspacing/data/map",  vspacingdata.map,  "builders.vspacing.data.map")
+storage.register("builders/vspacing/data/skip", vspacingdata.skip, "builders.vspacing.data.skip")
+
+do -- todo: interface.variables
+
+    vspacing.fixed = false
+
+    local map  = vspacingdata.map
+    local skip = vspacingdata.skip
+
+    local multiplier = C(S("+-")^0 * R("09")^1) * P("*")
+    local category   = P(":") * C(P(1)^1)
+    local keyword    = C((1-category)^1)
+    local splitter   = (multiplier + Cc(1)) * keyword * (category + Cc(false))
+
+    local k_fixed, k_flexible, k_category, k_penalty, k_order = variables.fixed, variables.flexible, "category", "penalty", "order"
+
+    -- This will change: just node.write and we can store the values in skips which
+    -- then obeys grouping
+
+    local fixedblankskip         = context.fixedblankskip
+    local flexibleblankskip      = context.flexibleblankskip
+    local setblankcategory       = context.setblankcategory
+    local setblankorder          = context.setblankorder
+    local setblankpenalty        = context.setblankpenalty
+    local setblankhandling       = context.setblankhandling
+    local flushblankhandling     = context.flushblankhandling
+    local addpredefinedblankskip = context.addpredefinedblankskip
+    local addaskedblankskip      = context.addaskedblankskip
+
+    local function analyze(str,oldcategory) -- we could use shorter names
+        for s in gmatch(str,"([^ ,]+)") do
+            local amount, keyword, detail = lpegmatch(splitter,s) -- the comma splitter can be merged
+            if not keyword then
+                report_vspacing("unknown directive %a",s)
+            else
+                local mk = map[keyword]
+                if mk then
+                    category = analyze(mk,category)
+                elseif keyword == k_fixed then
+                    fixedblankskip()
+                elseif keyword == k_flexible then
+                    flexibleblankskip()
+                elseif keyword == k_category then
+                    local category = tonumber(detail)
+                    if category then
+                        setblankcategory(category)
+                        if category ~= oldcategory then
+                            flushblankhandling()
+                            oldcategory = category
+                        end
+                    end
+                elseif keyword == k_order and detail then
+                    local order = tonumber(detail)
+                    if order then
+                        setblankorder(order)
+                    end
+                elseif keyword == k_penalty and detail then
+                    local penalty = tonumber(detail)
+                    if penalty then
+                        setblankpenalty(penalty)
+                    end
+                else
+                    amount = tonumber(amount) or 1
+                    local sk = skip[keyword]
+                    if sk then
+                        addpredefinedblankskip(amount,keyword)
+                    else -- no check
+                        addaskedblankskip(amount,keyword)
+                    end
+                end
+            end
+        end
+        return category
+    end
+
+    local pushlogger         = context.pushlogger
+    local startblankhandling = context.startblankhandling
+    local stopblankhandling  = context.stopblankhandling
+    local poplogger          = context.poplogger
+
+    function vspacing.analyze(str)
+        if trace_vspacing then
+            pushlogger(report_vspacing)
+            startblankhandling()
+            analyze(str,1)
+            stopblankhandling()
+            poplogger()
+        else
+            startblankhandling()
+            analyze(str,1)
+            stopblankhandling()
+        end
+    end
+
+    --
+
+    function vspacing.setmap(from,to)
+        map[from] = to
+    end
+
+    function vspacing.setskip(key,value,grid)
+        if value ~= "" then
+            if grid == "" then grid = value end
+            skip[key] = { value, grid }
+        end
+    end
+
+end
+
+-- implementation
+
+local trace_list, tracing_info, before, after = { }, false, "", ""
+
+local function nodes_to_string(head)
+    local current, t = head, { }
+    while current do
+        local id = current.id
+        local ty = nodecodes[id]
+        if id == penalty_code then
+            t[#t+1] = formatters["%s:%s"](ty,current.penalty)
+        elseif id == glue_code then -- or id == kern_code then -- to be tested
+            t[#t+1] = formatters["%s:%p"](ty,current)
+        elseif id == kern_code then
+            t[#t+1] = formatters["%s:%p"](ty,current.kern)
+        else
+            t[#t+1] = ty
+        end
+        current = current.next
+    end
+    return concat(t," + ")
+end
+
+local function reset_tracing(head)
+    trace_list, tracing_info, before, after = { }, false, nodes_to_string(head), ""
+end
+
+local function trace_skip(str,sc,so,sp,data)
+    trace_list[#trace_list+1] = { "skip", formatters["%s | %p | category %s | order %s | penalty %s"](str, data, sc or "-", so or "-", sp or "-") }
+    tracing_info = true
+end
+
+local function trace_natural(str,data)
+    trace_list[#trace_list+1] = { "skip", formatters["%s | %p"](str, data) }
+    tracing_info = true
+end
+
+local function trace_info(message, where, what)
+    trace_list[#trace_list+1] = { "info", formatters["%s: %s/%s"](message,where,what) }
+end
+
+local function trace_node(what)
+    local nt = nodecodes[what.id]
+    local tl = trace_list[#trace_list]
+    if tl and tl[1] == "node" then
+        trace_list[#trace_list] = { "node", formatters["%s + %s"](tl[2],nt) }
+    else
+        trace_list[#trace_list+1] = { "node", nt }
+    end
+end
+
+local function trace_done(str,data)
+    if data.id == penalty_code then
+        trace_list[#trace_list+1] = { "penalty", formatters["%s | %s"](str,data.penalty) }
+    else
+        trace_list[#trace_list+1] = { "glue", formatters["%s | %p"](str,data) }
+    end
+    tracing_info = true
+end
+
+local function show_tracing(head)
+    if tracing_info then
+        after = nodes_to_string(head)
+        for i=1,#trace_list do
+            local tag, text = unpack(trace_list[i])
+            if tag == "info" then
+                report_collapser(text)
+            else
+                report_collapser("  %s: %s",tag,text)
+            end
+        end
+        report_collapser("before: %s",before)
+        report_collapser("after : %s",after)
+    end
+end
+
+-- alignment box begin_of_par vmode_par hmode_par insert penalty before_display after_display
+
+local skipcodes = nodes.skipcodes
+
+local userskip_code              = skipcodes.userskip
+local lineskip_code              = skipcodes.lineskip
+local baselineskip_code          = skipcodes.baselineskip
+local parskip_code               = skipcodes.parskip
+local abovedisplayskip_code      = skipcodes.abovedisplayskip
+local belowdisplayskip_code      = skipcodes.belowdisplayskip
+local abovedisplayshortskip_code = skipcodes.abovedisplayshortskip
+local belowdisplayshortskip_code = skipcodes.belowdisplayshortskip
+local topskip_code               = skipcodes.topskip
+local splittopskip_code          = skipcodes.splittopskip
+
+local free_glue_node = free_node
+local discard, largest, force, penalty, add, disable, nowhite, goback, together = 0, 1, 2, 3, 4, 5, 6, 7, 8
+
+-- local function free_glue_node(n)
+--  -- free_node(n.spec)
+--     print("before",n)
+--     logs.flush()
+--     free_node(n)
+--     print("after")
+--     logs.flush()
+-- end
+
+function vspacing.snapbox(n,how)
+    local sv = snapmethods[how]
+    if sv then
+        local box = texbox[n]
+        local list = box.list
+        if list then
+            local s = list[a_snapmethod]
+            if s == 0 then
+                if trace_vsnapping then
+                --  report_snapper("box list not snapped, already done")
+                end
+            else
+                local ht, dp = box.height, box.depth
+                if false then -- todo: already_done
+                    -- assume that the box is already snapped
+                    if trace_vsnapping then
+                        report_snapper("box list already snapped at (%p,%p): %s",
+                            ht,dp,listtoutf(list))
+                    end
+                else
+                    local h, d, ch, cd, lines = snap_hlist("box",box,sv,ht,dp)
+                    box.height, box.depth = ch, cd
+                    if trace_vsnapping then
+                        report_snapper("box list snapped from (%p,%p) to (%p,%p) using method %a (%s) for %a (%s lines): %s",
+                            h,d,ch,cd,sv.name,sv.specification,"direct",lines,listtoutf(list))
+                    end
+                    box[a_snapmethod] = 0 --
+                    list[a_snapmethod] = 0 -- yes or no
+                end
+            end
+        end
+    end
+end
+
+local function forced_skip(head,current,width,where,trace)
+    if where == "after" then
+        head, current = insert_node_after(head,current,new_rule(0,0,0))
+        head, current = insert_node_after(head,current,new_kern(width))
+        head, current = insert_node_after(head,current,new_rule(0,0,0))
+    else
+        local c = current
+        head, current = insert_node_before(head,current,new_rule(0,0,0))
+        head, current = insert_node_before(head,current,new_kern(width))
+        head, current = insert_node_before(head,current,new_rule(0,0,0))
+        current = c
+    end
+    if trace then
+        report_vspacing("inserting forced skip of %p",width)
+    end
+    return head, current
+end
+
+-- penalty only works well when before skip
+
+local function collapser(head,where,what,trace,snap,a_snapmethod) -- maybe also pass tail
+    if trace then
+        reset_tracing(head)
+    end
+    local current, oldhead = head, head
+    local glue_order, glue_data, force_glue = 0, nil, false
+    local penalty_order, penalty_data, natural_penalty = 0, nil, nil
+    local parskip, ignore_parskip, ignore_following, ignore_whitespace, keep_together = nil, false, false, false, false
+    --
+    -- todo: keep_together: between headers
+    --
+    local function flush(why)
+        if penalty_data then
+            local p = new_penalty(penalty_data)
+            if trace then trace_done("flushed due to " .. why,p) end
+            head = insert_node_before(head,current,p)
+        end
+        if glue_data then
+            if force_glue then
+                if trace then trace_done("flushed due to " .. why,glue_data) end
+                head = forced_skip(head,current,glue_data.spec.width,"before",trace)
+                free_glue_node(glue_data)
+            elseif glue_data.spec.writable then
+                if trace then trace_done("flushed due to " .. why,glue_data) end
+                head = insert_node_before(head,current,glue_data)
+            else
+                free_glue_node(glue_data)
+            end
+        end
+        if trace then trace_node(current) end
+        glue_order, glue_data, force_glue = 0, nil, false
+        penalty_order, penalty_data, natural_penalty = 0, nil, nil
+        parskip, ignore_parskip, ignore_following, ignore_whitespace = nil, false, false, false
+    end
+    if trace_vsnapping then
+        report_snapper("global ht/dp = %p/%p, local ht/dp = %p/%p",
+            texdimen.globalbodyfontstrutheight, texdimen.globalbodyfontstrutdepth,
+            texdimen.bodyfontstrutheight, texdimen.bodyfontstrutdepth)
+    end
+    if trace then trace_info("start analyzing",where,what) end
+    while current do
+        local id = current.id
+        if id == hlist_code or id == vlist_code then
+            -- needs checking, why so many calls
+            if snap then
+                local list = current.list
+                local s = current[a_snapmethod]
+                if not s then
+                --  if trace_vsnapping then
+                --      report_snapper("mvl list not snapped")
+                --  end
+                elseif s == 0 then
+                    if trace_vsnapping then
+                        report_snapper("mvl %a not snapped, already done: %s",nodecodes[id],listtoutf(list))
+                    end
+                else
+                    local sv = snapmethods[s]
+                    if sv then
+                        -- check if already snapped
+                        if list and already_done(id,list,a_snapmethod) then
+                            local ht, dp = current.height, current.depth
+                            -- assume that the box is already snapped
+                            if trace_vsnapping then
+                                report_snapper("mvl list already snapped at (%p,%p): %s",ht,dp,listtoutf(list))
+                            end
+                        else
+                            local h, d, ch, cd, lines = snap_hlist("mvl",current,sv)
+                            if trace_vsnapping then
+                                report_snapper("mvl %a snapped from (%p,%p) to (%p,%p) using method %a (%s) for %a (%s lines): %s",
+                                    nodecodes[id],h,d,ch,cd,sv.name,sv.specification,where,lines,listtoutf(list))
+                            end
+                        end
+                    elseif trace_vsnapping then
+                        report_snapper("mvl %a not snapped due to unknown snap specification: %s",nodecodes[id],listtoutf(list))
+                    end
+                    current[a_snapmethod] = 0
+                end
+            else
+                --
+            end
+        --  tex.prevdepth = 0
+            flush("list")
+            current = current.next
+        elseif id == penalty_code then
+         -- natural_penalty = current.penalty
+         -- if trace then trace_done("removed penalty",current) end
+         -- head, current = remove_node(head, current, true)
+            current = current.next
+        elseif id == kern_code then
+            if snap and trace_vsnapping and current.kern ~= 0 then
+                report_snapper("kern of %p kept",current.kern)
+            end
+            flush("kern")
+            current = current.next
+        elseif id == glue_code then
+            local subtype = current.subtype
+            if subtype == userskip_code then
+                local sc = current[a_skipcategory]   -- has no default, no unset (yet)
+                local so = current[a_skiporder] or 1 -- has  1 default, no unset (yet)
+                local sp = current[a_skippenalty]    -- has no default, no unset (yet)
+                if sp and sc == penalty then
+                    if not penalty_data then
+                        penalty_data = sp
+                    elseif penalty_order < so then
+                        penalty_order, penalty_data = so, sp
+                    elseif penalty_order == so and sp > penalty_data then
+                        penalty_data = sp
+                    end
+                    if trace then trace_skip("penalty in skip",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                elseif not sc then  -- if not sc then
+                    if glue_data then
+                        if trace then trace_done("flush",glue_data) end
+                        head = insert_node_before(head,current,glue_data)
+                        if trace then trace_natural("natural",current) end
+                        current = current.next
+                    else
+                        -- not look back across head
+                        local previous = current.prev
+                        if previous and previous.id == glue_code and previous.subtype == userskip_code then
+                            local ps = previous.spec
+                            if ps.writable then
+                                local cs = current.spec
+                                if cs.writable and ps.stretch_order == 0 and ps.shrink_order == 0 and cs.stretch_order == 0 and cs.shrink_order == 0 then
+                                    local pw, pp, pm = ps.width, ps.stretch, ps.shrink
+                                    local cw, cp, cm = cs.width, cs.stretch, cs.shrink
+                                 -- ps = writable_spec(previous) -- no writable needed here
+                                 -- ps.width, ps.stretch, ps.shrink = pw + cw, pp + cp, pm + cm
+                                    previous.spec = new_gluespec(pw + cw, pp + cp, pm + cm) -- else topskip can disappear
+                                    if trace then trace_natural("removed",current) end
+                                    head, current = remove_node(head, current, true)
+                                --  current = previous
+                                    if trace then trace_natural("collapsed",previous) end
+                                --  current = current.next
+                                else
+                                    if trace then trace_natural("filler",current) end
+                                    current = current.next
+                                end
+                            else
+                                if trace then trace_natural("natural (no prev spec)",current) end
+                                current = current.next
+                            end
+                        else
+                            if trace then trace_natural("natural (no prev)",current) end
+                            current = current.next
+                        end
+                    end
+                    glue_order, glue_data = 0, nil
+                elseif sc == disable then
+                    ignore_following = true
+                    if trace then trace_skip("disable",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                elseif sc == together then
+                    keep_together = true
+                    if trace then trace_skip("together",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                elseif sc == nowhite then
+                    ignore_whitespace = true
+                    head, current = remove_node(head, current, true)
+                elseif sc == discard then
+                    if trace then trace_skip("discard",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                elseif ignore_following then
+                    if trace then trace_skip("disabled",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                elseif not glue_data then
+                    if trace then trace_skip("assign",sc,so,sp,current) end
+                    glue_order = so
+                    head, current, glue_data = remove_node(head, current)
+                elseif glue_order < so then
+                    if trace then trace_skip("force",sc,so,sp,current) end
+                    glue_order = so
+                    free_glue_node(glue_data)
+                    head, current, glue_data = remove_node(head, current)
+                elseif glue_order == so then
+                    -- is now exclusive, maybe support goback as combi, else why a set
+                    if sc == largest then
+                        local cs, gs = current.spec, glue_data.spec
+                        local cw, gw = cs.width, gs.width
+                        if cw > gw then
+                            if trace then trace_skip("largest",sc,so,sp,current) end
+                            free_glue_node(glue_data) -- also free spec
+                            head, current, glue_data = remove_node(head, current)
+                        else
+                            if trace then trace_skip("remove smallest",sc,so,sp,current) end
+                            head, current = remove_node(head, current, true)
+                        end
+                    elseif sc == goback then
+                        if trace then trace_skip("goback",sc,so,sp,current) end
+                        free_glue_node(glue_data) -- also free spec
+                        head, current, glue_data = remove_node(head, current)
+                    elseif sc == force then
+                        -- last one counts, some day we can provide an accumulator and largest etc
+                        -- but not now
+                        if trace then trace_skip("force",sc,so,sp,current) end
+                        free_glue_node(glue_data) -- also free spec
+                        head, current, glue_data = remove_node(head, current)
+                    elseif sc == penalty then
+                        if trace then trace_skip("penalty",sc,so,sp,current) end
+                        free_glue_node(glue_data) -- also free spec
+                        glue_data = nil
+                        head, current = remove_node(head, current, true)
+                    elseif sc == add then
+                        if trace then trace_skip("add",sc,so,sp,current) end
+                     -- local old, new = glue_data.spec, current.spec
+                        local old, new = writable_spec(glue_data), current.spec
+                        old.width   = old.width   + new.width
+                        old.stretch = old.stretch + new.stretch
+                        old.shrink  = old.shrink  + new.shrink
+                        -- toto: order
+                        head, current = remove_node(head, current, true)
+                    else
+                        if trace then trace_skip("unknown",sc,so,sp,current) end
+                        head, current = remove_node(head, current, true)
+                    end
+                else
+                    if trace then trace_skip("unknown",sc,so,sp,current) end
+                    head, current = remove_node(head, current, true)
+                end
+                if sc == force then
+                    force_glue = true
+                end
+            elseif subtype == lineskip_code then
+                if snap then
+                    local s = current[a_snapmethod]
+                    if s and s ~= 0 then
+                        current[a_snapmethod] = 0
+                        if current.spec.writable then
+                            local spec = writable_spec(current)
+                            spec.width = 0
+                            if trace_vsnapping then
+                                report_snapper("lineskip set to zero")
+                            end
+                        end
+                    else
+                        if trace then trace_skip("lineskip",sc,so,sp,current) end
+                        flush("lineskip")
+                    end
+                else
+                    if trace then trace_skip("lineskip",sc,so,sp,current) end
+                    flush("lineskip")
+                end
+                current = current.next
+            elseif subtype == baselineskip_code then
+                if snap then
+                    local s = current[a_snapmethod]
+                    if s and s ~= 0 then
+                        current[a_snapmethod] = 0
+                        if current.spec.writable then
+                            local spec = writable_spec(current)
+                            spec.width = 0
+                            if trace_vsnapping then
+                                report_snapper("baselineskip set to zero")
+                            end
+                        end
+                    else
+                        if trace then trace_skip("baselineskip",sc,so,sp,current) end
+                        flush("baselineskip")
+                    end
+                else
+                    if trace then trace_skip("baselineskip",sc,so,sp,current) end
+                    flush("baselineskip")
+                end
+                current = current.next
+            elseif subtype == parskip_code then
+                -- parskip always comes later
+                if ignore_whitespace then
+                    if trace then trace_natural("ignored parskip",current) end
+                    head, current = remove_node(head, current, true)
+                elseif glue_data then
+                    local ps, gs = current.spec, glue_data.spec
+                    if ps.writable and gs.writable and ps.width > gs.width then
+                        glue_data.spec = copy_node(ps)
+                        if trace then trace_natural("taking parskip",current) end
+                    else
+                        if trace then trace_natural("removed parskip",current) end
+                    end
+                    head, current = remove_node(head, current, true)
+                else
+                    if trace then trace_natural("honored parskip",current) end
+                    head, current, glue_data = remove_node(head, current)
+                end
+            elseif subtype == topskip_code or subtype == splittopskip_code then
+                if snap then
+                    local s = current[a_snapmethod]
+                    if s and s ~= 0 then
+                        current[a_snapmethod] = 0
+                        local sv = snapmethods[s]
+                        local w, cw = snap_topskip(current,sv)
+                        if trace_vsnapping then
+                            report_snapper("topskip snapped from %p to %p for %a",w,cw,where)
+                        end
+                    else
+                        if trace then trace_skip("topskip",sc,so,sp,current) end
+                        flush("topskip")
+                    end
+                else
+                    if trace then trace_skip("topskip",sc,so,sp,current) end
+                    flush("topskip")
+                end
+                current = current.next
+            elseif subtype == abovedisplayskip_code then
+                --
+                if trace then trace_skip("above display skip (normal)",sc,so,sp,current) end
+                flush("above display skip (normal)")
+                current = current.next
+                --
+            elseif subtype == belowdisplayskip_code then
+                --
+                if trace then trace_skip("below display skip (normal)",sc,so,sp,current) end
+                flush("below display skip (normal)")
+                current = current.next
+                --
+            elseif subtype == abovedisplayshortskip_code then
+                --
+                if trace then trace_skip("above display skip (short)",sc,so,sp,current) end
+                flush("above display skip (short)")
+                current = current.next
+                --
+            elseif subtype == belowdisplayshortskip_code then
+                --
+                if trace then trace_skip("below display skip (short)",sc,so,sp,current) end
+                flush("below display skip (short)")
+                current = current.next
+                --
+            else -- other glue
+                if snap and trace_vsnapping and current.spec.writable and current.spec.width ~= 0 then
+                    report_snapper("glue %p of type %a kept",current.spec.width,skipcodes[subtype])
+                --~ current.spec.width = 0
+                end
+                if trace then trace_skip(formatted["glue of type %a"](subtype),sc,so,sp,current) end
+                flush("some glue")
+                current = current.next
+            end
+        else
+            flush("something else")
+            current = current.next
+        end
+    end
+    if trace then trace_info("stop analyzing",where,what) end
+ -- if natural_penalty and (not penalty_data or natural_penalty > penalty_data) then
+ --     penalty_data = natural_penalty
+ -- end
+    if trace and (glue_data or penalty_data) then
+        trace_info("start flushing",where,what)
+    end
+    local tail
+    if penalty_data then
+        tail = find_node_tail(head)
+        local p = new_penalty(penalty_data)
+        if trace then trace_done("result",p) end
+        head, tail = insert_node_after(head,tail,p)
+    end
+    if glue_data then
+        if not tail then tail = find_node_tail(head) end
+        if trace then trace_done("result",glue_data) end
+        if force_glue then
+            head, tail = forced_skip(head,tail,glue_data.spec.width,"after",trace)
+            free_glue_node(glue_data)
+        else
+            head, tail = insert_node_after(head,tail,glue_data)
+        end
+    end
+    if trace then
+        if glue_data or penalty_data then
+            trace_info("stop flushing",where,what)
+        end
+        show_tracing(head)
+        if oldhead ~= head then
+            trace_info("head has been changed from %a to %a",nodecodes[oldhead.id],nodecodes[head.id])
+        end
+    end
+    return head, true
+end
+
+-- alignment after_output end box new_graf vmode_par hmode_par insert penalty before_display after_display
+-- \par -> vmode_par
+--
+-- status.best_page_break
+-- tex.lists.best_page_break
+-- tex.lists.best_size (natural size to best_page_break)
+-- tex.lists.least_page_cost (badness of best_page_break)
+-- tex.lists.page_head
+-- tex.lists.contrib_head
+
+local stackhead, stacktail, stackhack = nil, nil, false
+
+local function report(message,lst)
+    report_vspacing(message,count_nodes(lst,true),nodeidstostring(lst))
+end
+
+function vspacing.pagehandler(newhead,where)
+    -- local newhead = texlists.contrib_head
+    if newhead then
+        local newtail = find_node_tail(newhead) -- best pass that tail, known anyway
+        local flush = false
+        stackhack = true -- todo: only when grid snapping once enabled
+        for n in traverse_nodes(newhead) do -- we could just look for glue nodes
+            local id = n.id
+            if id ~= glue_code then
+                flush = true
+            elseif n.subtype == userskip_code then
+                if n[a_skipcategory] then
+                    stackhack = true
+                else
+                    flush = true
+                end
+            else
+                -- tricky
+            end
+        end
+        if flush then
+            if stackhead then
+                if trace_collect_vspacing then report("appending %s nodes to stack (final): %s",newhead) end
+                stacktail.next = newhead
+                newhead.prev = stacktail
+                newhead = stackhead
+                stackhead, stacktail = nil, nil
+            end
+            if stackhack then
+                stackhack = false
+                if trace_collect_vspacing then report("processing %s nodes: %s",newhead) end
+--~                 texlists.contrib_head = collapser(newhead,"page",where,trace_page_vspacing,true,a_snapmethod)
+                    newhead = collapser(newhead,"page",where,trace_page_vspacing,true,a_snapmethod)
+            else
+                if trace_collect_vspacing then report("flushing %s nodes: %s",newhead) end
+--~                 texlists.contrib_head = newhead
+            end
+        else
+            if stackhead then
+                if trace_collect_vspacing then report("appending %s nodes to stack (intermediate): %s",newhead) end
+                stacktail.next = newhead
+                newhead.prev = stacktail
+            else
+                if trace_collect_vspacing then report("storing %s nodes in stack (initial): %s",newhead) end
+                stackhead = newhead
+            end
+            stacktail = newtail
+         -- texlists.contrib_head = nil
+            newhead = nil
+        end
+    end
+    return newhead
+end
+
+local ignore = table.tohash {
+    "split_keep",
+    "split_off",
+ -- "vbox",
+}
+
+function vspacing.vboxhandler(head,where)
+    if head and not ignore[where] and head.next then
+    --  starttiming(vspacing)
+        head = collapser(head,"vbox",where,trace_vbox_vspacing,true,a_snapvbox) -- todo: local snapper
+    --  stoptiming(vspacing)
+    end
+    return head
+end
+
+function vspacing.collapsevbox(n) -- for boxes but using global a_snapmethod
+    local list = texbox[n].list
+    if list then
+    --  starttiming(vspacing)
+        texbox[n].list = vpack_node(collapser(list,"snapper","vbox",trace_vbox_vspacing,true,a_snapmethod))
+    --  stoptiming(vspacing)
+    end
+end
+
+-- We will split this module so a few locals are repeated. Also this will be
+-- rewritten.
+
+nodes.builders = nodes.builder or { }
+local builders = nodes.builders
+
+local actions = nodes.tasks.actions("vboxbuilders")
+
+function builders.vpack_filter(head,groupcode,size,packtype,maxdepth,direction)
+    local done = false
+    if head then
+        starttiming(builders)
+        if trace_vpacking then
+            local before = nodes.count(head)
+            head, done = actions(head,groupcode,size,packtype,maxdepth,direction)
+            local after = nodes.count(head)
+            if done then
+                nodes.processors.tracer("vpack","changed",head,groupcode,before,after,true)
+            else
+                nodes.processors.tracer("vpack","unchanged",head,groupcode,before,after,true)
+            end
+        else
+            head, done = actions(head,groupcode)
+        end
+        stoptiming(builders)
+    end
+    return head, done
+end
+
+-- This one is special in the sense that it has no head and we operate on the mlv. Also,
+-- we need to do the vspacing last as it removes items from the mvl.
+
+local actions = nodes.tasks.actions("mvlbuilders")
+
+local function report(groupcode,head)
+    report_page_builder("trigger: %s",groupcode)
+    report_page_builder("  vsize    : %p",tex.vsize)
+    report_page_builder("  pagegoal : %p",tex.pagegoal)
+    report_page_builder("  pagetotal: %p",tex.pagetotal)
+    report_page_builder("  list     : %s",head and nodeidstostring(head) or "<empty>")
+end
+
+function builders.buildpage_filter(groupcode)
+    local head, done = texlists.contrib_head, false
+ -- if head and head.next and head.next.id == hlist_code and head.next.width == 1 then
+ --     report_page_builder("trigger otr calculations")
+ --     free_node_list(head)
+ --     head = nil
+ -- end
+    if head then
+        starttiming(builders)
+        if trace_page_builder then
+            report(groupcode,head)
+        end
+        head, done = actions(head,groupcode)
+        stoptiming(builders)
+     -- -- doesn't work here (not passed on?)
+     -- tex.pagegoal = tex.vsize - tex.dimen.d_page_floats_inserted_top - tex.dimen.d_page_floats_inserted_bottom
+        texlists.contrib_head = head
+        return done and head or true
+    else
+        if trace_page_builder then
+            report(groupcode)
+        end
+        return nil, false
+    end
+end
+
+callbacks.register('vpack_filter',     builders.vpack_filter,     "vertical spacing etc")
+callbacks.register('buildpage_filter', builders.buildpage_filter, "vertical spacing etc (mvl)")
+
+statistics.register("v-node processing time", function()
+    return statistics.elapsedseconds(builders)
+end)
+
+-- interface
+
+commands.vspacing          = vspacing.analyze
+commands.vspacingsetamount = vspacing.setskip
+commands.vspacingdefine    = vspacing.setmap
+commands.vspacingcollapse  = vspacing.collapsevbox
+commands.vspacingsnap      = vspacing.snapbox