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diff --git a/tex/context/base/luat-cbk.lua b/tex/context/base/luat-cbk.lua
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+++ b/tex/context/base/luat-cbk.lua
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+if not modules then modules = { } end modules ['luat-cbk'] = {
+    version   = 1.001,
+    comment   = "companion to luat-lib.tex",
+    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
+    copyright = "PRAGMA ADE / ConTeXt Development Team",
+    license   = "see context related readme files"
+}
+
+--[[ldx--
+<p>Callbacks are the real asset of <l n='luatex'/>. They permit you to hook
+your own code into the <l n='tex'/> engine. Here we implement a few handy
+auxiliary functions.</p>
+--ldx]]--
+
+callbacks       = { }
+callbacks.stack = { }
+
+--[[ldx--
+<p>When you (temporarily) want to install a callback function, and after a
+while wants to revert to the original one, you can use the following two
+functions.</p>
+--ldx]]--
+
+function callbacks.push(name, func)
+    if not callbacks.stack[name] then
+        callbacks.stack[name] = { }
+    end
+    table.insert(callbacks.stack[name],callback.find(name))
+    callback.register(name, func)
+end
+
+function callbacks.pop(name)
+--  this fails: callback.register(name, table.remove(callbacks.stack[name]))
+    local func = table.remove(callbacks.stack[name])
+    callback.register(name, func)
+end
+
+--[[ldx--
+<p>The simple case is to remove the callback:</p>
+
+<code>
+callbacks.push('linebreak_filter')
+... some actions ...
+callbacks.pop('linebreak_filter')
+</code>
+
+<p>Often, in such case, another callback or a macro call will pop
+the original.</p>
+
+<p>In practice one will install a new handler, like in:</p>
+
+<code>
+callbacks.push('linebreak_filter', function(...)
+    return something_done(...)
+end)
+</code>
+
+<p>Even more interesting is:</p>
+
+<code>
+callbacks.push('linebreak_filter', function(...)
+    callbacks.pop('linebreak_filter')
+    return something_done(...)
+end)
+</code>
+
+<p>This does a one-shot.</p>
+--ldx]]--
+
+
+--[[ldx--
+<p>Callbacks may result in <l n='lua'/> doing some hard work
+which takes time and above all resourses. Sometimes it makes
+sense to disable or tune the garbage collector in order to
+keep the use of resources acceptable.</p>
+
+<p>At some point in the development we did some tests with counting
+nodes (in thsi case 121049).</p>
+
+<table>
+<tr><td>setstepmul</td><td>seconds</td><td>megabytes</td</tr>
+<tr><td>200</td><td>24.0</td><td>80.5</td</tr>
+<tr><td>175</td><td>21.0</td><td>78.2</td</tr>
+<tr><td>150</td><td>22.0</td><td>74.6</td</tr>
+<tr><td>160</td><td>22.0</td><td>74.6</td</tr>
+<tr><td>165</td><td>21.0</td><td>77.6</td</tr>
+<tr><td>125</td><td>21.5</td><td>89.2</td</tr>
+<tr><td>100</td><td>21.5</td><td>88.4</td</tr>
+</table>
+
+<p>The following code is kind of experimental. In the documents
+that describe the development of <l n='luatex'/> we report
+on speed tests. One observation is thta it sometimes helps to
+restart the collector.</p>
+--ldx]]--
+
+garbagecollector = { }
+
+do
+    local level = 0
+
+    collectgarbage("setstepmul", 165)
+
+    garbagecollector.trace = false
+    garbagecollector.tune  = false -- for the moment
+
+    function report(format)
+        if garbagecollector.trace then
+         -- texio.write_nl(string.format(format,level,status.luastate_bytes))
+            texio.write_nl(string.format(format,level,collectgarbage("count")))
+        end
+    end
+
+    function garbagecollector.update()
+        report("%s: memory before update: %s")
+        collectgarbage("restart")
+    end
+
+    function garbagecollector.push()
+        if garbagecollector.tune then
+            level = level + 1
+            if level == 1 then
+                collectgarbage("stop")
+            end
+            report("%s: memory after push: %s")
+        else
+            garbagecollector.update()
+        end
+    end
+
+    function garbagecollector.pop()
+        if garbagecollector.tune then
+            report("%s: memory before pop: %s")
+            if level == 1 then
+                collectgarbage("restart")
+            end
+            level = level - 1
+        end
+    end
+
+    function garbagecollector.cycle()
+        if garbagecollector.tune then
+            report("%s: memory before collect: %s")
+            collectgarbage("collect")
+            report("%s: memory after collect: %s")
+        end
+    end
+
+end
+