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+% language=uk
+
+\environment luametatex-style
+
+\startcomponent luametatex-preamble
+
+\startchapter[reference=preamble,title={The internals}]
+
+\topicindex{nodes}
+\topicindex{boxes}
+\topicindex{\LUA}
+
+This is a reference manual, not a tutorial. This means that we discuss changes
+relative to traditional \TEX\ and also present new functionality. As a
+consequence we will refer to concepts that we assume to be known or that might be
+explained later. Because the \LUATEX\ and \LUAMETATEX\ engines open up \TEX\
+there's suddenly quite some more to explain, especially about the way a (to be)
+typeset stream moves through the machinery. However, discussing all that in
+detail makes not much sense, because deep knowledge is only relevant for those
+who write code not possible with regular \TEX\ and who are already familiar with
+these internals (or willing to spend time on figuring it out).
+
+So, the average user doesn't need to know much about what is in this manual. For
+instance fonts and languages are normally dealt with in the macro package that
+you use. Messing around with node lists is also often not really needed at the
+user level. If you do mess around, you'd better know what you're dealing with.
+Reading \quotation {The \TEX\ Book} by Donald Knuth is a good investment of time
+then also because it's good to know where it all started. A more summarizing
+overview is given by \quotation {\TEX\ by Topic} by Victor Eijkhout. You might
+want to peek in \quotation {The \ETEX\ manual} too.
+
+But \unknown\ if you're here because of \LUA, then all you need to know is that
+you can call it from within a run. If you want to learn the language, just read
+the well written \LUA\ book. The macro package that you use probably will provide
+a few wrapper mechanisms but the basic \lpr {directlua} command that does the job
+is:
+
+\starttyping
+\directlua{tex.print("Hi there")}
+\stoptyping
+
+You can put code between curly braces but if it's a lot you can also put it in a
+file and load that file with the usual \LUA\ commands. If you don't know what
+this means, you definitely need to have a look at the \LUA\ book first.
+
+If you still decide to read on, then it's good to know what nodes are, so we do a
+quick introduction here. If you input this text:
+
+\starttyping
+Hi There
+\stoptyping
+
+eventually we will get a linked lists of nodes, which in \ASCII\ art looks like:
+
+\starttyping
+H <=> i <=> [glue] <=> T <=> h <=> e <=> r <=> e
+\stoptyping
+
+When we have a paragraph, we actually get something:
+
+\starttyping
+[localpar] <=> H <=> i <=> [glue] <=> T <=> h <=> e <=> r <=> e <=> [glue]
+\stoptyping
+
+Each character becomes a so called glyph node, a record with properties like the
+current font, the character code and the current language. Spaces become glue
+nodes. There are many node types that we will discuss later. Each node points
+back to a previous node or next node, given that these exist. Sometimes
+multiple characters are represented by one glyphs, so one can also get:
+
+\starttyping
+[localpar] <=> H <=> i <=> [glue] <=> Th <=> e <=> r <=> e <=> [glue]
+\stoptyping
+
+And maybe some characters get positioned relative to each other, so we might
+see:
+
+\starttyping
+[localpar] <=> H <=> [kern] <=> i <=> [glue] <=> Th <=> e <=> r <=> e <=> [glue]
+\stoptyping
+
+It's also good to know beforehand that \TEX\ is basically centered around
+creating paragraphs and pages. The par builder takes a list and breaks it into
+lines. At some point horizontal blobs are wrapped into vertical ones. Lines are
+so called boxes and can be separated by glue, penalties and more. The page
+builder accumulates lines and when feasible triggers an output routine that will
+take the list so far. Constructing the actual page is not part of \TEX\ but done
+using primitives that permit manipulation of boxes. The result is handled back to
+\TEX\ and flushed to a (often \PDF) file.
+
+The \LUATEX\ engine provides hooks for \LUA\ code at nearly every reasonable
+point in the process: collecting content, hyphenating, applying font features,
+breaking into lines, etc. This means that you can overload \TEX's natural
+behaviour, which still is the benchmark. When we refer to \quote {callbacks} we
+means these hooks. The \TEX\ engine itself is pretty well optimized but when you
+kick in much \LUA\ code, you will notices that performance drops. Don't blame and
+bother the authors with performance issues. In \CONTEXT\ over 50\% of the time
+can be spent in \LUA, but so far we didn't get many complaints about efficiency.
+
+Where plain \TEX\ is basically a basic framework for writing a specific style,
+macro packages like \CONTEXT\ and \LATEX\ provide the user a whole lot of
+additional tools to make documents look good. They hide the dirty details of font
+management, language demands, turning structure into typeset results, wrapping
+pages, including images, and so on. You should be aware of the fact that when you
+hook in your own code to manipulate lists, this can interfere with the macro
+package that you use. Each successive step expects a certain result and if you
+mess around to much, the engine eventually might bark and quit. It can even
+crash, because testing everywhere for what users can do wrong is no real option.
+
+When you read about nodes in the following chapters it's good to keep in mind
+their commands that relate to then. Here are a few:
+
+\starttabulate[|l|l|p|]
+\DB command              \BC node          \BC explanation \NC \NR
+\TB
+\NC \prm {hbox}          \NC \nod {hlist} \NC horizontal box \NC \NR
+\NC \prm {vbox}          \NC \nod {vlist} \NC vertical box with the baseline at the bottom \NC \NR
+\NC \prm {vtop}          \NC \nod {vlist} \NC vertical box with the baseline at the top \NC \NR
+\NC \prm {hskip}         \NC \nod {glue}  \NC horizontal skip with optional stretch and shrink \NC \NR
+\NC \prm {vskip}         \NC \nod {glue}  \NC vertical skip with optional stretch and shrink \NC \NR
+\NC \prm {kern}          \NC \nod {kern}  \NC horizontal or vertical fixed skip \NC \NR
+\NC \prm {discretionary} \NC \nod {disc}  \NC hyphenation point (pre, post, replace) \NC \NR
+\NC \prm {char}          \NC \nod {glyph} \NC a character \NC \NR
+\NC \prm {hrule}         \NC \nod {rule}  \NC a horizontal rule \NC \NR
+\NC \prm {vrule}         \NC \nod {rule}  \NC a vertical rule \NC \NR
+\NC \prm {textdirection} \NC \nod {dir}   \NC a change in text direction \NC \NR
+\LL
+\stoptabulate
+
+Text (interspersed with macros) comes from an input medium. This can be a file,
+token list, macro body cq.\ arguments, \ some internal quantity (like a number),
+\LUA, etc. Macros get expanded. In the process \TEX\ can enter a group. Inside
+the group, changes to registers get saved on a stack, and restored after leaving
+the group. When conditionals are encountered, another kind of nesting happens,
+and again there is a stack involved. Tokens, expansion, stacks, input levels are
+all terms used in the next chapters. Don't worry, they loose their magic once you
+use \TEX\ a lot. You have access to most of the internals and when not, at least
+it is possible to query some state we're in or level we're at.
+
+When we talk about packing it can mean two things. When \TEX\ has consumed some
+tokens that represent text the next can happen. When the text is put into a so
+called \type {\hbox} it (normally) first gets hyphenated, next ligatures are
+build, and finally kerns are added. Each of that stages can be overloaded using
+\LUA\ code. When these three stages are finished, the dimension of the content is
+calculated and the box gets its width, height and depth. What happens with the
+box depends on what macros do with it.
+
+The other thing that can happen is that the text starts a new paragraph. In that
+case some (directional) information is put in front, indentation is prepended and
+some skip appended at the end. Again the three stages are applied but this time,
+afterwards, the long line is broken into lines and the result is either added to
+the content of a box or to the main vertical list (the running text so to say).
+This is called par building. At some point \TEX\ decides that enough is enough
+and it will trigger the page builder. So, building is another concept we will
+encounter. Another example of a builder is the one that turns an intermediate
+math list into something typeset.
+
+Wrapping something in a box is called packing. Adding something to a list is
+described in terms of contributing. The more complicated processes are wrapped
+into builders. For now this should be enough to enable you to understand the next
+chapters. The text is not as enlightening and entertaining as Don Knuths books,
+sorry.
+
+\stopchapter
+
+\stopcomponent