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+
+Document: TeX in a nutshell
+
+Author: Petr Olsak, <petr@olsak.net>
+        Czech Technical University in Prague
+
+This software is in public domain.
+
+The pure TeX features are described here.
+The main goal of this document is its brevity.
+I hope that this document can be useful for somebody 
+who is looking for information about basics of TeX.
+
+Files:
+tex-nutshell.tex ... source file
+tex-nutshell.pdf ... the document
+
+Version:
+See the end of the document.
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+\useoptex % We are using OpTeX, no LaTeX
+
+\report
+\fontfam[pagella]
+
+\emergencystretch=2em
+\hbadness=2100
+\catcode`<=13
+\def<#1>{{\def\ { }\iindex{:#1}\iis {:#1} {$\langle\hbox{\it#1}\rangle$}}%
+   $\def\,{\hskip1.5pt plus.3pt minus1.2pt}\,\langle\hbox{\it#1}\rangle\,$}
+\everyintt={\catcode`<=13 \Blue}
+\onlyrgb
+\addto\_titfont\Blue
+\def\Green{\setrgbcolor{0 .8 0}}
+
+\def\myunderline#1{\vtop{\hbox{#1}\kern-\prevdepth \kern2pt \hrule}}
+\adef/#1.{\myunderline{#1}} 
+\adef|#1;{{\Red#1}}
+\toksapp\everyintt{\catcode`\/=13 \catcode`\|=13}
+\catcode`\/=12 \catcode`\|=12
+\adef\%#1\par{{\adef\%{\%}\Green\%#1}\par}
+\catcode`\%=14
+\toksapp\everytt{\catcode`\%=13}
+
+\def\*{*}
+\def\c#1{$_{#1}$}
+
+\activettchar`
+\enquotes
+
+% to index macros:
+\def\i #1 {\ii .#1 \iis .#1 {{\code{\\#1}}}}
+\def\x`{\bgroup\_setverb\xx}
+\bgroup \lccode\string`\.=\string`\` \lowercase{\egroup \def\xx #1#2.{\i #2 \egroup `#1#2.}}
+\def\y`{\bgroup\_setverb\yy}
+\def\yy #1#2={\i #2 \egroup `#1#2=}
+
+% Hyperlinks
+\hyperlinks\Red\Green
+\fnotelinks\Magenta\Magenta
+
+
+
+\tit \TeX/ in a nutshell
+
+\author Petr Olšák
+
+The pure \TeX/ features are described here, no features provided by
+macro extensions. Only the last section gives a summary of plain \TeX/
+macros.
+
+The main goal of this document is its brevity. So features are described
+only roughly and sometimes inaccurately here. If you need to know more then you can read
+free available books, for example 
+\ulink[https://eijkhout.net/texbytopic/texbytopic.html]{\TeX/ by topic} 
+or
+\ulink[http://petr.olsak.net/tbn.html]{\TeX/book naruby}.
+Try to type `texdoc texbytopic` in your system.
+
+\ii OpTeX \ulink[http://petr.olsak.net/optex]{\OpTeX/} manual 
+supposes that the user knows the basics principles of \TeX/ itself. 
+If you are converting from \LaTeX/ to \OpTeX/ for example\fnote
+{Congratulations to your decision:-)}
+then you may welcome a summary
+document that presents these basic principles because \LaTeX/ manuals
+typically don't distinguish between \TeX/ features and features specially
+implemented by \LaTeX/ macros. 
+
+\notoc\nonum\sec Table of contents
+
+\maketoc \outlines0
+
+
+\sec[termi] Terminology
+
+The main principle of \TeX/ is that its input files can be a mix
+of the material which could be printed and \ii control~sequence {\em control sequences}
+which gives a setting for build-in algorithms of \TeX/ or gives a special message
+to \TeX/ what to do with the inputted material.
+
+Each control sequence (typically a word prefixed by a backslash) has its 
+\ii meaning~of~control~sequence {\em meaning}. 
+There are four types of meanings of control sequences:
+
+\begitems
+* the control sequence can be a \ii register {\em register}, it means it represents a variable which
+  is able to keep a value. There are \ii primitive/register {\em primitive registers}. Their values influence
+  behavior of build-in algorithm (e.g. \i hsize `\hsize`, \i parindent `\parindent`,
+  \i hyphenpenalty `\hyphenpenalty`). On the other hand \ii decalared/register {\em declared registers} 
+  are used by macros (e.g. \i medskipamount `\medskipamount` used in plain \TeX/ 
+  or \i ttindent `\ttindent` 
+  used by \ii OpTeX \OpTeX/).
+* the control sequence can be a \ii primitive/command {\em primitive command}, it runs a build-in
+  algorithm (e.g. \i def `\def` declares a macro, \i halign `\halign` runs algorithm for
+  tables, \i hbox `\hbox` creates a box in typesetting output).
+* the control sequences can be a \ii character/constant {\em character constant} 
+  (declared by \i chardef `\chardef`
+  or \i mathchardef `\mathchardef` primitive command) or a font selector (declared
+  by \i font `\font` primitive command).
+* the control sequence can be a \ii macro {\em macro}. When it is read then it is
+  replaced by its \ii replacement/text {\em replacement text} in the input queue. If there are more
+  macros in the replacement text, all macros are replaced too. It is so called 
+  \ii expansion/process {\em expansion process} which ends on the level of text to be printed or 
+  primitive commands or registers or character constants or font selectors.
+\enditems
+
+Example. When \TeX/ reads:
+\begtt
+\def\TeX{T\kern-.1667em\lower.5ex\hbox{E}\kern-.125emX}
+\endtt
+in a macro file then the `\def` primitive command saves the information that
+\i TeX `\TeX` is a control sequence with meaning \"macro", the replacement text is
+declared here and it is a mix of a material to be typeset: `T`, `E` and `X` 
+and primitive commands 
+\i kern `\kern`, \i lower `\lower`, \i hbox `\hbox` 
+with their parameters in given syntax. Each
+primitive command has declared syntax, for example, `\kern` must be followed
+by dimension specification in the format \"decimal number followed by a unit".
+More about this primitive syntax is in the sections~\ref[reg], \ref[expand] and~\ref[main].
+
+When a control sequence `\TeX` with meaning \"macro" occurs in the
+input stream then it is \ii expansion {\em expanded} to its replacement text, i.e the sequence of
+typesetting material and primitive commands in our case of `\TeX` macro. 
+The logo \TeX/ is printed as a result of this processing.
+
+None of the control sequences have their definitive meaning. The control sequence
+could change its meaning by re-defining it as a new macro (using `\def`), 
+redeclaring it as an arbitrary object in \TeX/ (using \i let `\let`) etc.
+When you re-define a primitive control sequence then the access to its value
+or build-in algorithm is lost. This is a reason why \ii OpTeX \OpTeX/ macros duplicate
+all primitive sequences (\i hbox `\hbox` and `\_hbox`) with the same meaning and use
+only \"private" control sequences (prefixed by `_`. So, user can re-define 
+`\hbox` without the loss of the primitive command `\_hbox`.
+
+\sec Formats, engines
+
+\TeX/ is able to start without any macros preloaded in so-called \ii ini-TeX/state {\em ini-\TeX/
+state} (the `-ini` option on the command line must be used). It knows only Cca 300
+primitive registers and primitive commands at this state.\fnote
+{Roughly speaking, if you know all these 300 primitive objects and all syntax
+of primitive commands and all build-in algorithms then you know all about \TeX.
+But starting to produce common documents from this primitive level without
+macro support is nearly impossible.}
+When ini-\TeX/ reads macro files then new control sequences are
+declared as macros, declared registers, character constants or font
+selectors. The primitive command \i dump `\dump` saves the binary image of the \TeX/ memory
+(with newly declared control sequences) to the \ii format,format/file {\em format file} (`.fmt`
+extension). 
+
+The original intention of existence format files was to prepare a
+collection of macro declarations, register settings and to load default fonts and
+to dump this information to a file for later use. Such collection typically
+declares macros for the markup of documents and for typesetting design. This is
+the reason why we call these files {\em format files}: they give a format
+of documents on the output side and declares markup rules for document source files.
+
+When \TeX/ is started without `-ini` option then it tries to load a prepared
+format file into its memory and to continue with reading more macros or a real
+document (or both). The starting point is at the place where \i dump `\dump` was
+processed during ini-\TeX/ state. If the format file is not specified
+explicitly (by `-fmt` option on the command line) then \TeX/ tries to read the
+format file with the same name which is used for running \TeX. For example
+`tex document` runs \TeX, it loads the format `tex.fmt` and reads the
+`document.tex`. Or `latex document` runs \TeX, it loads the format `latex.fmt`
+and reads the `document.tex`. 
+
+The `tex.fmt` is format file dumped when \ii plain~TeX/macros {\em plain \TeX/ macros}\fnote
+{Plain \TeX/ macros were made by \ii Knuth/Donald Donald Knuth, the author of \TeX. It is a
+set of basic macros and settings which is used (more or less) as a subset of
+all other macro packages.} 
+were read and `latex.fmt` is format file dumped when \ii LaTeX/macros {\em \LaTeX/ macros} were read.
+This is typically done when a \TeX/ distribution is installed without any user
+intervention. So, the user can run `tex document` or `latex document` without
+worry that these typical format files exist.
+
+From this point of view, \LaTeX/ is nothing more than a format of \TeX/,
+i.e.~a collection of macro declarations and register settings.
+
+A typical \TeX/ distributions have four common \ii TeX/engines {\em \TeX/ engines}, i.e.~programs
+they implement classical \TeX/ algorithms with various extensions: 
+
+\begitems
+* \TeX/ -- only classical \TeX/ algorithms by Donald Knuth, 
+* \ii pdfTeX pdf\TeX -- an extension supporting PDF output directly and
+             micro-typographical features,
+* \ii XeTeX Xe\TeX/ -- an extension supporting Unicode and PDF output, 
+* \ii luaTeX lua\TeX/ -- an extension supporting Lua programming, Unicode,
+            micro-typographical features and PDF output.
+\enditems
+
+Each of them are able to run in ini-\TeX/ state or with a format file. For
+example the command `luatex -ini macros.ini` starts lua\TeX/ at ini-\TeX/ state,
+read `macros.ini` file and final `\dump` command is supposed here to create
+a format `macros.fmt`. Then user can use the command `luatex -fmt macros document` to
+load `macros.fmt` and to process the `document.tex`.
+Or the command `luatex document` processes lua\TeX/ with `document.tex` and 
+with `luatex.fmt` which is a little extension of plain \TeX/ macros. Another example:
+`lualatex document` runs lua\TeX/ with `lualatex.fmt`. It is a format with
+\LaTeX/ macros for lua\TeX/ engine. Final example: 
+`optex document` runs Lua\TeX/ with `optex.fmt` which is
+a format with \ii OpTeX \ulink[http://petr.olsak.net/optex]{\OpTeX/ macros}.
+
+\sec Searching data
+
+If \TeX/ needs to read something from the file system (for example the
+primitive command \i input `\input<file name>` or 
+\i font `\font<font selector>=<file name>` is used) 
+then the rule
+\"first wins" is applied. \TeX/ looks at the current directory first or
+somewhere to the \TeX/ installation second. The behavior in the second step
+depends on the used \TeX/ distribution. For example 
+\ii TeXlive \ulink[https://www.tug.org/texlive]{\TeX/live} programs are
+linked with a \ii kpathsea {\em kpathsea} library and they do the following: Search the given
+file in the current directory, then in \ii texmf~tree \code{~/texmf} tree (data are saved by
+the user here), then in `texmf-local` tree (data are saved by the system administrator
+here, they are not removed when \TeX/ distribution is upgraded), 
+then in `texmf-var` tree (data are saved automatically by programs from
+\TeX/ distribution here) and then in `texmf-dist` tree (data from \TeX live
+distribution). Each directory tree can be divided into sub-trees: first level
+`tex`, `fonts`, `doc` etc., second level is divided by \TeX/ engines or font types etc.,
+more levels are typically organized to keep a clarity.
+New files in the current directory on in \code{~/texmf} tree are found without 
+doing something more, but new files in other places have to be registered by `texhash`
+program (\TeX/ distribution does automatically this during its installation).
+
+
+\sec Processing the input
+
+The lines from input files are transformed by the \ii tokenizer {\em tokenizer} first.
+It reads input lines and generates a sequence of tokens. The main goals of
+tokenizer is
+
+\begitems
+* It converts each control sequence to a single token characterized by its name.
+* Another input material is tokenized as \"one token per character".
+* The continuous sequence of more spaces is transformed into one space token.
+* The end of the line is transfromed into space token, so the paragraph text
+  can continue in a next input line and one space token is between the last word
+  on the previous line and the first word on the next line.
+* Comment character `%` is ignored and all the text after it to the end of line
+  is ignored too.
+* Spaces from the begining of each line are ignored. Thus, you can
+  use arbitrary indentation in your source file without change of the result.
+* Each empty line (or line with only spaces) is transformed to the token
+  \i par `\par`. This token has primitive meaning: \"finalize the current paragraph".
+  This implies the general rule in \TeX/ source files: paragraphs are terminated 
+  by empty lines. 
+\enditems
+
+The behavior of the tokenizer is not definitive. Tokenizer works with a table of
+category codes. Any change of category codes of characters
+(done by \i catcode {\let\,=\relax`\catcode`\code{`}`\<character>=<code>`} 
+primitive command) influences tokenizer processing. For
+example, the verbatim environment is declared using setting all characters to
+normal meaning.
+
+By default, there are the following characters with special meaning. Tokenizer
+converts them or sets them as special tokens used in syntactical rules in
+\TeX/ later. The corresponding category codes are mentioned here as an index of the
+character.
+
+\begitems 
+* `\`\c0\quad -- starts completion of a control sequence by the tokenizer.
+* `{`\c1 and `}`\c2\quad -- opens and close group or have special syntactical meaning.
+  The main syntactical rule is: each subsequence of tokens treated by macros
+  or primitive commands must have these pairs o tokens balanced. There is no
+  exception. Tokenizer treats them as special tokens with meaning
+  \"opening character" and \"closing character".
+* `%`\c{14}\quad -- comment character, removed by the tokenizer.
+* `$`\c3, `&`\c4, `#`\c6, `^`\c7, `_`\c8, `~`\c{13}\quad -- 
+  tokenizer treats them as a special tokens with meaning:
+  \ii math/mode/selector \"math-mode selector\c3", 
+  \ii table/separator \"table separator\c4", \ii parameter/prefix \"parameter prefix for
+  macros\c6", \ii superscript/prefix \"superscript prefix in math\c7", 
+  \ii subscript prefix \"subscript prefix in math\c8"
+  \ii active/character \"active character\c{13}" 
+  (the active character `~` is defined as no-breakable space in all typical formats).
+* Letters and other characters are tokenized as \"letter character\c{11}" or
+  \"other character\c{12}".
+\enditems
+
+If you need to print these special characters you can use 
+\ii -percent,-at,-dollar,-hash
+`\%`, `\&`, `\$`, `\#` or `\_`. These five control sequences are declared as
+\"print this character" in all typical \TeX/ formats.
+Another possibility is to use a verbatim environment (it depends on used format) 
+Last alternative: you can use \i csstring {\let\,=\relax`\csstring\<character>`} 
+in lua\TeX/, because lua\TeX/ disposes
+with the primitive command `\csstring` which converts `\<character>`
+to `<character>`\c{12}.
+
+The \ii active~character \"active character\c{13}"
+can be declared by \i catcode `\catcode`\code{`}`\<character>=13`. 
+Such `<character>` behaves like `<control sequence>`. For example, you can
+define it by
+\i def `\def<character>{...}`.
+If a `<control sequence>` is
+listed in this document, the active character is meant too.
+{\emergencystretch=2em\par}
+
+Each control sequence is built by the tokenizer starting from `\`\c0. Its name is
+a continuous sequence of letters\c{11} finalized by first non-letter. 
+Note that \OpTeX/ sets
+`_` as letter\c{11}, thus control sequence names can include this character.
+\LaTeX/ sets the `@` as letter\c{12} when reading styles and macro files.
+You can look to such files and you will see many such characters inside
+private control sequence names declared by \LaTeX/ macros.
+
+If the first character after `\`\c0 is not letter\c{\ne11} then control sequence is
+finalized with only this character in its name. So called 
+\ii one~character/control~sequence {\em one-character control sequence} is created. Other control sequences 
+are \ii multiletter/control~sequence {\em multiletter control sequences}.
+
+Spaces {\char9251}\c{10} after multi-letter control sequences are ignored, so the space can be used
+as a terminating character of the control sequence. Another characters used
+immediately after a control sequences are not ignored. So `\TeX !` and
+`\TeX!` gives the same result: the control sequence `\TeX` followed
+immediately by `!`\c{12}.
+
+Tokenizer's output (a sequence of tokens) goes to \ii expand/processor {\em expand processor}
+and its output goes to \ii main/processor {\em main processor} of \TeX. The expand processor
+performs expansions of macros or a primitive command which is working at expand
+processor level. See a summary of such commands in the section~\ref[expand]. 
+
+Main processor performs assignment of registers, declares macros by \i def `\def`
+primitive command and runs all primitive commands at the main processor level.
+Moreover, it creates the typesetting output as described in the next
+section.
+
+The very important difference between \TeX/ and other programs is that there are no
+strings, only sequences of tokens. We can return to the example
+`\def\TeX{...}` above in the section~\ref[termi]. 
+The token \i def `\def` is a control sequence with meaning \"declare a macro".
+It gets the following token \i TeX `\TeX` and
+declares it as a macro with replacement text, which is the sequence of tokens:
+
+\medskip
+\bgroup \def\b#1{\kern1pt\lower2ex\llap{\c{#1}\kern-1.5pt}}
+   \def`#1`#2,{\frame{\strut\tt\Blue\string#1}\b{#2}}\catcode`\{=12 \catcode`\}=12 \indent
+`T`11, `\kern`, `-`12, `.`12, `1`12, `6`12, `6`12, `7`12, `e`11, `m`11,
+`\lower`, `.`12, `5`12, `e`11, `x`11, `\hbox`, `{`1, `E`11, `}`2, 
+`\kern`, `-`12, `.`12, `1`12, `2`12, `5`12, `e`11, `m`11, `X`11,
+\egroup
+\medskip
+
+If you are thinking like \TeX/ then you must forget the term \"string"
+because all texts in \TeX/ are preprocessed by the tokenizer when input lines are
+read and only sequences of tokens are manipulated inside \TeX/.
+
+Tokenizer converts two `^`\c7`^`\c7 characters followed by ASCII uppercase
+letter to the Ctrl-letter ASCII code. For example `^^M` is Ctrl-M (carriage
+return). It converts two `^`\c7`^`\c7 followed by two hexadecimal digits 
+(`0123456789abcdef`) to one-byte code, for example, `^^0d` is Ctrl-M too 
+because it has code 13. Moreover, tokenizer of \XeTeX/ or lua\TeX/ converts
+`^`\c7`^`\c7`^`\c7`^`\c7 followed by four hexadecimal digits or 
+`^`\c7`^`\c7`^`\c7`^`\c7`^`\c7`^`\c7 followed by six hexadecimal digits to one
+character with given Unicode.
+
+
+\sec Vertical and horizontal modes
+
+When the main processor creates the typesetting output then it alternates
+between vertical and horizontal mode. It starts in \ii vertical/mode,@ {\em vertical mode}: all
+materials are put vertically below in this mode. For example
+\i hbox `\hbox{a}\hbox{b}\hbox{c}` creates c below b and b below a in vertical mode.
+
+If there is something incompatible with vertical mode principle: a special
+command working only in horizontal mode or a character itself 
+then main processor switches to \ii horizontal/mode,@ {\em horizontal mode}: it opens 
+an unlimited horizontal data line for
+typesetting material and puts material next to each other. For example
+\i hbox `\hbox{a}\hbox{b}\hbox{c}` creates abc in horizontal mode.
+
+When an empty line is scanned then tokenizer creates \i par `\par` token here and
+if the main processor is in horizontal mode, the `\par` command finalizes the
+paragraph. More exactly it returns to the vertical mode, 
+it breaks the horizontal data line filled in previous horizontal mode 
+to parts with the \i hsize `\hsize` width. These parts are completed as 
+\ii box {\em boxes} and they are put one below second in the vertical mode. So, a
+paragraph of \i hsize `\hsize` width is created.
+
+Repeatedly: if there is something incompatible with current vertical mode
+(typically a character), then the horizontal mode is opened and all characters
+(and spaces between them) are put to the horizontal data line. When empty line
+is scanned then `\par` command is started and the horizontal 
+data line is broken to the lines of \i hsize `\hsize` width and next paragraph 
+is completed.
+
+In the vertical mode, the material is cumulated in a vertical data column
+called \ii main/vertical/list {\em main vertical list}.
+If the height of this material is greater than \i vsize `\vsize` then its part with
+maximal `\vsize` height is completed as a \ii page/box {\em page box} and shipped to 
+\ii output/routine {\em output routine}. A programmer or designer can declare a design of pages
+using macros in the output routine: header, footer, pagination, 
+the position of the main page box etc. The output routine completes the main page 
+box with another material declared in the output routine and the result is
+shipped out as one page of the document. The main processor continues in
+the vertical mode with the rest of the unused material in the main vertical list.
+Then it can switch to the horizontal mode if a character occurs etc...
+
+The plain \TeX/ macro \i bye `\bye` (or primitive command \i end `\end`\fnote
+{\LaTeX/ format re-defines this primitive control sequence `\end` to another
+meaning which follows the logic of \LaTeX/'s markup rules.}) 
+starts the last
+`\par` command, finalizes the last paragraph (if any), completes the last page
+box, puts it to the output routine, finalizes the last page in it and \TeX/
+is terminated.
+
+There are \ii internal/vertical/mode {\em internal vertical mode} and 
+\ii internal/horizontal/mode {\em internal horizontal mode}.
+They are activated when the main processor is typesetting a material 
+inside \i vbox `\vbox{...}` or \i hbox `\hbox{...}` primitive commands. More about boxes is in
+sections~\ref[boxes] and~\ref[main]. 
+
+Understanding of switching between modes is very important for \TeX/ users.
+There are primitive commands which are context dependent on the current mode.
+For example, \i par `\par` primitive command (generated by an empty line) does nothing
+in vertical mode but it finalizes paragraph in horizontal mode and it causes
+an error in math mode. Or \i kern `\kern` primitive command creates a vertical space in
+vertical mode or horizontal space in horizontal mode.
+
+The following primitive commands used in the vertical mode starts the
+horizontal mode: the first character of the paragraph (most common
+situation) or \i indent `\indent`, \i noindent `\noindent`, \i hskip `\hskip` 
+(and its alternatives),
+\i vrule `\vrule`\fnote {The list is not fully completed, but most important commands
+are mentioned here.} and plain \TeX/ macro \i leavevmode `\leavevmode`.
+When the horizontal mode is opened, the indentation of the \i parindent `\parindent` width 
+is included. The exception is only if the horizontal mode is started by the
+\i noindent `\noindent`, then the paragraph has no indentation.
+
+The following primitive commands used in the horizontal mode finalizes the
+paragraph and returns to the vertical mode: \i par `\par`, \i vskip `\vskip` (and its
+alternatives), \i hrule `\hrule`, \i end `\end` and plain \TeX/ macro \i bye `\bye`.
+
+\sec Groups in \TeX/
+
+Each assignment to registers, declaration macros or font selecting is local
+in groups. When the current group ends then the assignments done inside the
+group are forgotten and the values when this group was opened are restored.
+The groups can be delimited by `{` and `}` pair or by \i begingroup `\begingroup` and 
+\i endgroup `\endgroup` primitive commands or by 
+\i bgroup `\bgroup` and \i egroup `\egroup` control
+sequences declared by plain \TeX. 
+For example, plain \TeX/ declares the macros \i rm `\rm` (selects roman font),
+\i bf `\bf` (selects bold font) and \i it `\it` (selects italics) and it initializes by
+\i rm `\rm` font. User can write:
+
+\begtt
+The roman font is here {\it here is italics} and the roman font continues.
+\endtt
+%
+Not only fonts but all registers are set locally inside a group. The macro
+designer can declare a special environments with font selecting and with
+more special typographical parameters in groups.
+
+The following example is a test of understanding vertical and horizontal
+modes switching. 
+
+\begtt
+{\hsize=5cm This is the first paragraph which should be formatted 
+            to 5 cm width.}
+
+But it is not true...
+\endtt
+%
+Why the example above does not create the paragraph with a 5 cm width?
+The empty line \i par (`\par`) command is placed {\em after} the group is finished, so the
+\i hsize `\hsize` parameter has its previous value at the time when the paragraph is
+completed, no value 5\,cm. The value of \i hsize `\hsize` register\fnote
+{and about twenty other registers which declare the paragraph design}
+is used when the paragraph is
+completed, no at the beginning of the paragraph. This is the reason why
+macro programmers put explicitly \i par `\par` command to macros before the local
+environment is finished by the end of the group. Our example should look like
+
+\begtt
+{\hsize=5cm This is the first ... to 5 cm width.\par}
+\endtt
+
+
+\sec[boxes] Box, kern, penalty, glue
+
+You can look at one character, say the `y`. It is represented by three
+dimensions: \ii height height (above baseline), \ii depth depth (below baseline) 
+and \ii width width.
+Suppose that there are more characters printed in horizontal mode and
+completed to a line of a paragraph. This line has its height equal to
+the maximal height of characters inside it, it has the depth equal to maximal
+depth of all characters inside it and it has its width. Such a sequence of
+characters encapsulated to one typesetting element
+with its height, depth and width is
+called \ii box {\em box}. Boxes are placed next to each other (from left to
+right\fnote{There is an exception for special languages.})
+in horizontal mode or one above second in vertical mode. 
+
+The boxes can include individual characters or spaces or boxes. The boxes can
+include more boxes. Paragraph lines are boxes. The page box includes paragraph
+lines (boxes). The finalized page with a header, page box, pagination,
+etc. is a box and it is shipped out to the PDF page. Understanding boxes is
+necessary for macro programmers and designers.
+
+You can create an individual box by the primitive command
+\i hbox `\hbox{<horizontal material>}`
+or \i vbox `\vbox{<vertical material>}`. The `<horizontal material>` is completed in
+internal horizontal mode and `<vertical material>` in internal vertical
+mode. Both cases open a group, create the material in a specified
+mode and closes the group, where all settings are local.
+
+The `<horizontal material>` can include individual characters, boxes,
+horizontal \ii glue {\em glues} or \ii kern {\em kerns}. The glue is a special term for
+stretchable or shrinkable and possible breakable spaces and the kern is a 
+term used for fixed nonbreakable spaces.
+
+The `<vertical material>` can include boxes, vertical glues or kerns. No
+individual characters. If you put an individual character in the 
+vertical mode (for example in the \i vbox `\vbox`)
+then the horizontal mode is opened. At the end of \i vbox `\vbox`\fnote 
+{before the \i vbox `\vbox` group is closed}
+or when \i par `\par` command is invoked, the opened paragraph is finished (with
+current \i hsize `\hsize` width) and the resulting lines are vertically placed
+inside the \i vbox `\vbox`. 
+
+The completed boxes are unbreakable and they are treaded as a single object in
+the surrounding printed material. 
+
+The line boxes of the paragraph have the fixed width \i hsize `\hsize`, so there must
+be something stretchable or shrinkable in order to get desired fixed width
+of lines. Typically the spaces between words have this feature\fnote {When
+the microtypographical 
+feature \i pdfadjustspacing `\pdfadjustspacing` is activated, then not
+only spaces are stretchable and shrinkable but individual characters are
+slightly deformed (by invisible amount) too.} These spaces have declared
+their \ii default/size/of~space {\em default size}, their 
+\ii stretchability {\em stretchability} and their 
+\ii shrinkability {\em shrinkability} in the font metric data of currently used font.
+
+You can place such glue explicitly by the primitive command:
+
+\begtt \catcode`<=13
+\hsize <default size> plus<stretchability> minus<shrinkability>
+for example:
+\hsize 10pt plus5pt minus2.5pt
+\endtt
+%
+This example places the glue with 10\,pt default size, stretchable to
+15\,pt\fnote
+{It can be stretchable ad absurdum (more than 15\,pt) but with very
+considerable \ii badness {\em badness} calculated by \TeX/ whenever glues are stretched 
+or shrank.}
+and shrinkable to 7.5\,pt as its minimal size.
+All glues in one line are stretched or shrank equally but with weights given
+from their stretchability/shrinkability values.
+
+You can do experiments of this feature if you say \i hbox `\hbox to<size>{...}`.
+Then the \i hbox `\hbox` is created with a given width. Probably, the glues inside
+this \i hbox `\hbox` must be stretched or shrank. You can see in the log
+that the total \ii badness {\em badness} is calculated, it represents the amount of a 
+\"force" used to all glue included in such \i hbox `\hbox`.
+
+An infinitely stretchable (to an arbitrary positive value) or shrinkable 
+(to an arbitrary negative value) glue
+can exist. This glue is stretched/shrank and other glues with
+finite amounts of stretching or shrinking keep their default size in such
+case.
+You can put infinitely stretchable/\penalty0shrinkable 
+\ii glue glue using the reserved unit \ii fil `fil`
+in \i hskip `\hskip` command, for example
+the command \i hskip `\hskip 0pt plus 1fil` means zero default size but infinitely stretchable.
+There is a shortcut for such glue: \i hfil `\hfil`. When you type
+`\hbox to\hsize{\hfil <text>\hfil}` then the `<text>` is centered.
+But if the `<text>` is wider than \i hsize `\hsize` then \TeX/ reports an 
+\ii overfull/box `overfull \hbox`. If you want to center a wide `<text>` too, you can use 
+\i hss `\hss` instead \i hfil `\hfil`. The \i hss `\hss` primitive command is equal to
+`\hskip 0pt plus1fil minus1fil`. The `<text>` printed by
+`\hbox to\hsize{\hss<text>\hss}` is now centered in its arbitrary size.
+
+A glue created with \ii fill `fill` stretchability or shrinkability (double ell)
+is infinitely more stretchable or shrinkable than glues with only \ii fil `fil` unit.
+So, glue with \ii fill `fill` are stretched or shrank and glues with only `fil` in
+the same box keep their default size. For example, a macro declares centering
+a `<text>` by 
+\i hbox `\hbox to\hsize{\hss <text>\hss}` and user can create the `<text>` in the form
+\i hfill `\hfill <real text>`. Then `<real text>` is printed flushed right because
+\i hfill `\hfill` is a shortcut to \i hskip `\hskip0pt plus1fill` and has greater priority than
+glues with only \ii fil `fil` unit. 
+
+Common usage is \i hbox `\hbox to0pt{<text>\hss}` or `\hbox to0pt{\hss<text>}`.
+The box with zero width is created and the text overlaps this dimension to
+right (first example) or to left (second example). \ii plain~TeX Plain \TeX/ declares
+macros for these cases: \i rlap `\rlap{<text>}` or \i llap `\llap{<text>}`.
+
+The last line of each paragraph is finalized by a glue of type \i hfil `\hfil` by
+default. When you write \i hfill `\hfill <object>` in vertical mode (`<object>` is
+something like a table, image or whatever else in the box) then `<object>` is
+flushed right, because the paragraph is started by the `\hfill` space but
+finalized only by \i hfil `\hfil` space. If you type 
+\i noindent `\noindent\hfil <object>` then
+the <object> is centered. And putting only `<object>` places it to the left
+side because the common left side is default placement rule in the vertical
+mode.
+
+The same principles concerned with horizontal glues are applicable for vertical
+modes where glues are created by \i vskip `\vskip` commands instead `\hskip`. You can
+write \i vbox `\vbox to<size>{...}` and do experiments.
+
+If a glue is put to the horizontal data line in horizontal mode and
+paragraph breaking algorithm decides about the suitable breakpoints for creating
+lines with desired width \i hsize `\hsize`, then each glue is potentially breakable
+point. Each glue can be preceded by a \ii penalty {\em penalty} value (created by
+\i penalty `\penalty` primitive) in the typical range
+-10000 to 10000. The paragraph breaking algorithm gets a penalty if it decides
+to break line in the glue with given penalty value preceded. If no penalty
+is declared for a given glue, then it is the same as a penalty equal to zero.
+The penalty value 10000 or more
+means \"impossible to break". The negative penalty means a bonus for paragraph
+breaking algorithm. The penalty -10000 or less means \"you must break here".
+
+The paragraph breaking algorithm tries to find an optimum of breakpoints
+positions concerning all penalties, to all badnesses of all created lines and to
+many more values not mentioned here in this brief document.
+The analogical optimal breakpoint is found in the vertical material when \TeX/
+breaks it into pages.
+
+The concept \"box, penalty, glue" with the optimum-fit breaking
+algorithms makes \TeX/ unique between many other typesetting software.
+
+
+\sec Syntactical rules
+
+The primitive commands can get their parameters written after it. These
+parameters must suit syntactical rules given for
+each primitive command. Some parameters are optional. For example 
+\i hskip `\hskip<dimen> /plus<stretchability>. /minus<shrinkability>.` means that
+the parameter `<dimen>` must follow (it must suit syntactical rules for
+dimensions, see section \ref[reg]) then the optional parameter prefixed by keyword 
+\ii plus `plus` can follow
+and then the optional parameter prefixed by \ii minus `minus` can follow.
+We denote the optional parameters by underline in this document.
+{\emergencystretch=2em\par}
+
+The \ii keyword {\em keywords} (typically prefixes some parameters) may have optional spaces
+around them.
+
+If the syntactical rule mentions the pair `{`, `}` then these characters are
+not definitive: other characters may be tokenized with this special meaning but it
+is not common. The text between this pair must be \ii balabced/text {\em balanced} with respect to this
+pair. For example the syntactic rule \i message `\message{<text>}` supposes that 
+`<text>` must not be `ab{cd` but `ab{c{}}d` is allowed for instance.
+
+By default, all parameters read by primitive commands are got from input
+stream tokenized and fully expanded by the expand processor. But
+sometimes, when \TeX/ reads parameters for a primitive command, the expand
+processor is deactivated. We denote these parameters by red color. For
+example \i let `\let|<control sequence>=<token>;` means that these parameters
+processed by `\let` command are not expanded.
+
+Whenever a syntactical rule mentions the \ii equal/sign `=` character (see the previous example
+with \i let `\let` command, then this is the equal sign tokenized as a normal character
+and it is optional. The syntactical rule allows to omit it. Optional spaces
+are allowed around this equal sign.
+
+The concept of the optional parameters of primitive commands 
+(terminated if something different from
+the keyword follows) may bring troubles if a macro programmer
+forget to terminate an uncomplete parameter text by \x`\relax` command
+(`\relax` does nothing but it can terminate a list of optional parameters of
+the previous command). 
+Suppose, for example, that `\mycoolspace` is defined 
+by `\def\mycoolspace{\penalty42\hskip2mm}`. If an user write
+`first\mycoolspace plus second` then \TeX/ reports the error
+`missing` `number,` `treated` `as` `zero` in the position of `s` character
+and appends: \code{<to be read again> s}. An user who is unfamiliar with
+\TeX/ primitive commands and their parameters is totally lost. The correct
+definition looks like: `\def\mycoolspace{\penalty42\hskip2mm\relax}`.
+
+\sec[def] Principles of macros
+
+Macros can be declared by \i def `\def` primitive command (or `\edef`, `\gdef`,
+`\xdef` commands, see below). The syntax is
+`\def|<control sequence>/<parameters>.{<replacement text>};`.
+
+The `<parameters>` are a sequence of formal parameters of the declared macro
+written in the form `#1`, `#2`, etc.
+They must be numbered from one and incremented by one.
+The maximal number of declared parameters is nine. 
+These parameters can be used in the `<replacement text>`. This specifies the
+place where the real parameter is positioned when the macro is expanded. For
+example:
+
+\begtt
+\def\test #1{here is "#1".}
+\test A        % expands to: here is "A".
+\def\swap #1#2{#2#1}
+\swap AB       % expands to: BA
+\test {param}  % expands to: here is "param".
+\swap A{param} % expands to: paramA
+\endtt
+%
+Note that there are two possibilities of how to write real macro
+parameters when a macro is in use. The parameter is one token by default but
+if there is `{<something>}` then the parameter is `<something>`. The braces
+here are delimiters for the real parameter.
+
+The example above shows a declaration of \ii unseparated/parameter,@ {\em unseparated parameters}. The
+parameters were declared by `#1` or `#1#2` with no text appended to such
+declaration. But there is another possibility. 
+Each formal parameter can have a text appended in its declaration, so
+the general syntax of declaration of formal parameters is
+`#1/<text1>.#2/<text2>.;` etc. If such `<text>` is appended then we say that
+the parameter is \ii separated/parameter,@ {\em separated} or 
+\ii delimited/parameter,@ {\em delimited} by text.
+The same delimiter must be used when the macro is in use. For example
+
+\begtt
+\def\Test #1#2..#3 {first "#1", second "#2", third "#3".}
+\Test ABC..DEF G % expands to: first "A", second "BC", third "DEF".
+                 % the letter G follows after expansion.
+\endtt
+%
+In the example above the `#1` parameter is unseparated (one token is read as
+a real parameter if not used the syntax `{<parameter>}`). The `#2` parameter
+is delimited by two dots and the `#3` parameter is delimited by space.
+
+There should be a `<text0>` immediately before `#1` in the parameter
+declaration. This means that the declared macro must be used with the same <text0>
+immediately appended. If not, \TeX/ reports the error.
+General rule for declaration a macro with three parameters should be:
+\i def
+`\def|<control sequence>/<text0>.#1/<text1>.#2/<text2>.#3/<text3>.{<replacement text>};`.
+
+The rule \"everything must be balanced" is applied to separated parameters
+too. It means that `\Test AB{C..DEF G}.. H` from the example above reads
+`B{C..DEF G}` to the `#2` parameter and the `#3` parameter is empty 
+because the space (the delimiter of `#3` parameter) immediately follows two dots.
+
+The separated parameter could bring a potential problem when the user forgot the
+delimiter or the delimiter is specified incorrectly. Then \TeX/ reports the
+error. This error is reported when the first \i par `\par` is scanned to the
+parameter (probably generated from an empty line). If you really want to scan
+to the parameter more paragraphs including `\par` between them then you can
+use \i long `\long` prefix before `\def`. For example `\long\def\scan#1\stop{...}`
+reads the parameter of the `\scan` macro to the `\stop` control sequence
+and this parameter could include more paragraphs. 
+If the delimiter is missing when \i long `\long` defined macro is processed then
+\TeX/ reports the error at the end of the file.
+
+When a real parameter of a macro is scanned then the expand processor is deactivated.
+When the `<replacement text>` is processed then the expand processor works
+normally. It means that if parameters are used in `<replacement text>` then
+they are expanded here.
+
+If a macro declaration is used inside `<replacement text>` of another macro
+then the number of `#` must be doubled for inner declaration. Example:
+
+\begtt
+\def\defmacro#1#2{%
+   \def#1##1 ##2 {##1 says: #1 ##2.}%
+}
+\defmacro \hello  {hello}  % expands to \def\hello#1 #2 {#1 says: hello #2.}
+\defmacro \goobye {good bye}
+\hello   Jane Eric         % expands to: Jane says: hello Eric.
+\goodbye Eric John         % expands to: Eric says: good bye John.
+\endtt
+
+Note the `%` characters used in the `\defmacro` definition. They mask
+the end of lines. If you don't use them then the space tokens are here (generated
+by tokenizer at the end of each line). The `<replacement text>` of `\defmacro` will be
+`<space>\def#1...{...}<space>` in such case. Each usage of `\defmacro` 
+generates two unwanted spaces. It is not a problem if `\defmacro` is used in
+the vertical mode because spaces are ignored in this mode. But 
+if `\defmacro` is used in horizontal mode then
+these spaces are printed\fnote
+{More precisely, they are transformed into horizontal glues used between words.}.
+
+The macro declaration behaves as another assignment, so the information about
+such declaration is lost if it is used in a group and the group is left.
+But you can use \i global `\global` prefix before \i def `\def` or the primitive
+\i gdef `\gdef`. Then the assignment is global regardless of groups.
+
+When `\def` or `\gdef` is processed then `<replacement text>` is read with
+deactivated expand processor. We have alternatives \i edef `\edef` (expanded def)
+and \i xdef `\xdef` (global expanded def) which read their `<replacement text>`
+expanded by expand processor. The summary of `\def` syntax is:
+
+\begtt \catcode`\|=13 \catcode`\/=13 \catcode`\<=13 \Blue 
+\def|<control sequence>/<parameters>.{<replacement text>};  % local assignment
+\gdef|<control sequence>/<parameters>.{<replacement text>}; % global assignment
+\edef|<control sequence>/<parameters>.{;<replacement text>} % local assignment
+\xdef|<control sequence>/<parameters>.{;<replacement text>} % global assignment
+\endtt   
+
+You can set \i tracingmacros `\tracingmacros=2` and you can see to log file how the macros are expanded.
+
+\sec Math modes
+
+The `$`\c3`<math text>$`\c3 specifies a math formula inside the line of the
+paragraph. It processes the `<math text>` in a group and in 
+\ii internal/math/mode,math/mode/internal {\em internal math mode}. 
+The `$`\c3`$`\c3`<math text>$`\c3`$`\c3 generates a separate line with math 
+formula(s). It processes the `<math text>` in a group and in 
+\ii display/math/mode,math/mode/display {\em dislay math mode}. 
+
+The fonts in math mode are selected by very specific manner which is independent 
+on current text font. Six different math object are automatically detected
+in math mode: \x`\mathord` (normal material), \x`\mathop` (big operators),
+\x`\mathbin` (binary operators), \x`\mathrel` (relations), \x`\mathopen` (open
+brackets), \x`\mathclose` (close brackets), \x`\mathpunct` (punctuations). They
+can be processed in four styles \x`\displaystyle` (default in the display mode),
+\x`\textstyle` (default in the internal math mode), \x`\scriptstyle` (used for
+indexes or exponents, more small text) and \x`\scriptscriptstyle` 
+(used in indexes of indexes, smaller text).
+
+The math algorithms were implemented to \TeX/ by its author with very care.
+All typographical traditions of math typesetting were taken into account.
+There are three chapters about math typesetting in his \TeX/book. Moreover,
+there is the detailed appendix G with exact specification of generating math
+formulas here. This topic is unfortunately out of the frame of this short
+text.
+
+There is a good a piece of news: all formats (including \LaTeX/) take default \TeX/
+syntax for `<math text>`. So, \LaTeX/ manuals or \LaTeX/ documents
+serves a good source if you want to get to know the rules of math typesetting
+by \TeX. There is only one significant difference. Fractions are constructed at
+the primitive level by the \x`\over` primitive:
+`{<nominator>\over<denominator>}` but \LaTeX/ uses a macro \x`\frac` in the
+syntax `\frac{<nominator>}{<denominator>}`. Plain \TeX/ users (including the
+author of \TeX/) prefer the syntax which follows the
+principle \"how a human reads the formula". On the other hand, the 
+\x`\frac` syntax is derived from machine languages. You can define the
+\x`\frac` macro by  `\def\frac#1#2{{#1\over#2}}` if you want.
+
+
+\sec[reg] Registers
+
+\ii register There are four types of registers used in \TeX:
+
+\begitems \def\!{\kern-1pt}
+* \ii counter/type/register {\em Counters}, their values are integer numbers. Counters are declared by 
+  \i newcount `\newcount|<register>;`\fnote
+  {The declarators \x`\newcount`, \x`\newdimen`, \x`\newskip` and \x`\newtoks`
+   are plain \TeX/ macros used in all known \TeX/ formats. They provides
+   `<address>` allocation and uses the  
+   `\count<address>`\!, `\dimen<address>`\!, `\skip<address>` and `\toks<address>` 
+   \TeX/ registers. The \x`\countdef`, \x`\dimendef`, \x`\skipdef` and \x`\toksdef` 
+   primitive commands are used internally.}
+   or they are primitive registers (\x`\linepenalty` for example).
+   \TeX/ interprets primitive commands which represent an integer from
+   an internal table as counter type register too 
+   (examples: \x`\catcode`\code{`A}, \x`\lccode`\code{`A}).
+* \ii dimen/type/register {\em Dimen type}, their values are dimensions. They are declared by
+  \i newdimen `\newdimen|<register>;` or they are primitive registers (\x`\hsize`, for example).
+   \TeX/ interprets primitive commands which represent a dimension
+   value as dimen type register too (example: \i wd `\wd0`).
+* \ii glue/type/register {\em Glue type}, their values are triples like in general 
+  \x`\hskip` parameters.
+  They can be declared by \i newskip `\newskip|<register>;` or they are primitive
+  registers (\x`\abovedisplayskip` for example).\fnote
+  {Very similar {muglue type} for math glues exists too 
+   but it is not described in this text.}
+* \ii token/type/register {\em Tokens lists}, their values are sequences of tokens. They are
+  declared by \i newtoks `\newtoks|<register>;` or they are primitive registers
+  (\x`\everypar` for example). 
+\enditems
+
+The following example shows, how registers are declared, how the value is
+saved to the register and how to print the value of the register. 
+
+\tmpnum=42 \tmpdim=-13cm \skip0=10mm plus 12mm minus1fil \toks0={abCd ef}
+\begtt
+\newcount  \mynumber
+\newdimen  \mydimension
+\newskip   \myskip
+\newtoks   \mytoks
+\mynumber = 42
+\mydimension = -13cm
+\myskip = 10mm plus 12mm minus1fil
+\mytoks = {abCd ef}
+To print these vaules use the primitive command "the":
+\the\mynumber, \the\mydimension, \the\myskip, \the\mytoks.
+\bye
+\endtt
+%
+This example prints: To print these values use the primitive command "the":
+\the\tmpnum, \the\tmpdim, \the\skip0, \the\toks0. Note that the human
+readable dimensions are converted to typographical points~(pt).
+
+The general syntactic rule for storing values to registers is 
+`<register>=<value>` where the equal sign is optional and it can be surrounded by
+optional spaces. Syntactic rules for each type of `<value>` depending on
+type of the register (i.\,e.\ `<number>`, `<dimen>`, `<skip>` and `<toks>`) follows.
+
+\begitems \let\_aboveliskip=\relax 
+* The `<number>` could be 
+\begitems \style -
+* a register of counter type;
+* a character constant declared by \x`\chardef` or \x`\mathchardef` primitive command. 
+* an integer decimal number (with optional `+` or `-` prefixed)
+* {\let\,=\relax `"<hexa number>`} where `<hexa number>` can include digits
+  `0123456789ABCDEF`\,;
+* {\let\,=\relax`'<octal number>`} where `<octal number>` can include digits 
+  `01234567`\,;
+* {\let\,=\relax \code{`}`<character>`} (the prefix is the reverse single quote \code{`}). 
+  It returns the code of the `<character>`. Examples:
+  \code{`}`A` or one-character control sequence \code{`\\A}).
+  Both examples represent the number 65. Unicode of the character 
+  are taken here if lua\TeX/ or Xe\TeX/ are used;
+* \i numexpr `\numexpr<num. expression>`.\fnote
+  {This is a feature of e\TeX/ extension. It is implemented in pdf\TeX, Xe\TeX/ and lua\TeX.}
+  The `<num. expression>` uses operators `+`, `-`, `*` and \code{/} and
+  brackets `(`, `)` in normal sense. The operands are `<number>`s. It
+  is terminated by something incompatible with
+  the syntactic rule of `<num. expression>` or by `\relax`.
+  If the result is non-integer, then it is rounded (no truncated).
+\enditems
+* The `<dimen>` could be
+\begitems \style -
+* a register of dimen type or counter type;
+* a decimal number with an optional decimal point (and optional `+` or `-`
+  prefixed) followed by `<dimen unit>`. The `<dimen unit>` is \ii pt `pt` (point)\fnote 
+  {1\,pt = 1/72.27\,in $\doteq$ 0.35\,mm\,;\ 1\,pc = 12\,pt\,;\
+   1\,bp = 1/72\,in\,;\ 1\,dd $\doteq$ 1.07\,pt\,;\ 1\,cc = 12\,dd\,;\
+   1\,sp = $2^{-16}$\,pt = \TeX/ accuracy.}
+  or \ii mm `mm` or \ii cm `cm` or \ii in `in` or 
+  \ii bp `bp` (big point) or \ii dd `dd` (Didot point) or \ii pc `pc` (pica) or 
+  \ii cc `cc` (cicero) or \ii sp `sp` or \ii em `em` (quad of current font) or
+  \ii ex `ex` (ex~height of current font) or a register of dimen type;  
+* \i dimexpr `\dimexpr<dimen expression>`.
+  The `<dimen expression>` uses operators `+`, `-`, `*` and \code{/} and
+  brackets `(`, `)` in normal sense. The operands of `+` and `-` are
+  `<dimen>`s, the operators of `*` or \code{/} are the pair `<dimen>` and
+  `<number>` (in this order). The `<dimen expression>`
+  is terminated by something what is incompatible with
+  syntactic rule of `<dimen expression>` or by `\relax`.
+\enditems
+* The `<skip>` could be:
+\begitems \style -
+* a register of glue type or dimen type or counter type;
+* `<dimen>/plus<generalized dimen>. /minus<generalized dimen>.`. The 
+  `<generalized dimen>` is the same as `<dimen>`, but normal `<dimen unit>` 
+   or pseudo-unit `fil` or `fill` or `filll` can be used.
+\enditems
+* The `<toks>` could be
+\begitems \style -
+* `/<expandafters>.{|<text>};`. The `<expandafters>` is typically a sequence of
+  `\expandafter` primitive commands (zero or more). The `<text>` is
+  scanned without expansion but the exception can be given by 
+  `<expandafters>`.
+\enditems
+\enditems
+
+\removelastskip
+The main processor reads input tokens (from the output of activated or
+deactivated expand processor) in two contexts: 
+\ii do/something/context,context/do/something {\em do something} or {\em
+read parameters}. By default it is in the context {\em do something}. When a
+primitive which allows a parameters is read, main processor reads the
+parameters in the context \ii read/parameters/context,context/read/parameters {\em read parameters}.
+
+Whenever the main processor reads a register in the context
+{\em do something} it assumes that an assignment of a value to the register
+is declared here. The following text (equal sign and `<value>`) is read in the context
+{\em read parameters}. If the following text isn't
+compliant to the appropriate syntactic rule, \TeX/ reports an error.
+
+Examples of register manipulations:
+
+\begtt
+\newcount\mynumber \newdimen\mydimension \newdimen\myskip
+\hsize = .7\hsize  % see the rule for <dimen>, unit could be a register 
+\hoffset = \dimexpr 10mm - (\parindent + 1in) \relax % usage of \dimexpr
+\myskip = 10pt plus15pt minus 3pt
+\mydimen = \myskip    % the information "plus15pt minus 3pt" is lost
+\mynumber = \mydimen  % \mynumber = 10*2^16  because \mydimen = 10*2^16 sp
+\endtt
+%
+Each dimension is saved internally as an integer multiple of `sp` unit in
+\TeX. When we need a conversion `<dimen>` $\to$ `<number>`, then simply the
+internal unit `sp` is omitted.
+
+The summary of most commonly used primitive registers including their default
+value given by plain \TeX/ follows.
+
+\begitems  \rightskip=0pt plus1fil
+* \y`\hsize=6.5in`,
+  \y`\vsize=8.9in`
+  are paragraph width and page height.
+
+* \y`\hoffset=0pt`,
+  \y`\voffset=0pt` give left margin and top margin of the page. They are
+  calculated from \ii page/origin {\em page origin} which is defined by coordinates
+  \y`\pdfvorigin=1in` and \y`\pdfhorigin=1in` measured from left upper corner of
+  the page.
+
+* \y`\parindent=20pt` is the indentation of the first line of each paragraph.
+
+* \y`\parfillskip=0pt plus 1fil` is horizontal glue added to the last line of the
+  paragraph.
+
+* \y`\leftskip=0pt`, \y`\rightskip=0pt`. Glues added to each line in the
+  paragraph from the left and the right side. If the stretchability is decared here,
+  then the paragraph is ragged left/right.
+
+* \y`\parskip=0pt plus 1pt` is vertical space between paragraphs.
+
+* \y`\baselineskip=12pt`,
+  \y`\lineskiplimit=0pt`,
+  \y`\lineskip=1pt`.
+  \ii baselineskiprule The {\em `\baselineskip` rule} says: 
+  Two consecutive lines in vertical list have baseline distance given 
+  by \x`\baselineskip` by default. The appropriate real glue is inserted
+  between the lines.
+  But if this real glue (between boxes) is less than \x`\lineskiplimit` 
+  then only \x`\lineskip` between boxes is inserted.
+
+* \y`\topskip=10pt` is the distance between top of page box and the baseline of
+  the first line.
+
+* \y`\linepenalty=10`,
+  \y`\hyphenpenalty=50`,
+  \y`\exhyphenpenalty=50`,
+  \y`\binoppenalty=700`,
+  \y`\relpenalty=500`,
+  \y`\clubpenalty=150`,
+  \y`\widowpenalty=150`,
+  \y`\displaywidowpenalty=50`,
+  \y`\brokenpenalty=100`,
+  \y`\predisplaypenalty=10000`,
+  \y`\postdisplaypenalty=0`,
+  \y`\interlinepenalty=0`,
+  \y`\floatingpenalty=0`,
+  \y`\outputpenalty=0`.
+  These penalties are put to various places in the vertical or horizontal
+  list. Most important are \x`\clubpenalty` (below the first line of a paragraph)
+  and \x`\widowpenalty` (before last line of paragraph). Typographical rules
+  give us to set these register to 10000 (no page break is allowed here).
+
+* \y`\looseness=0` allows to create a \"suboptimal" paragraph. The page-building
+  algorithm tries to builds the paragraph with \x`\loosenes` lines more than
+  the optimal solution. If the \x`\tolerance` has not sufficiently large value
+  then this setting is simply ignored. It is reset to zero after each
+  paragraph is completed.
+
+* \y`\spaceskip=0pt`,
+  \y`\xspaceskip=0pt`. If non-negative they are used as glues between words.
+  Default values are read from the font metric data of the current font.
+
+* \y`\pretolerance=100`,
+  \y`\tolerance=200`, \y`\emergencystretch=0pt`
+  \y`\doublehyphendemerits=10000`,
+  \y`\finalhyphendemerits=5000`,
+  \y`\adjdemerits=10000`,
+  \y`\hfuzz=0.1pt`,
+  \y`\vfuzz=0.1pt`
+are parameters for paragraph building algorithm not described here in
+detail.
+
+* \y`\hbadness=1000`,
+  \y`\vbadness=1000`. \TeX/ reports a warning about \iid badness on the terminal
+  and to the log file if it is greater than these values. The warning has the form
+  \ii underfull/box `underfull` `\hbox` or `underfull \vbox`. The value `100`
+  means that the `plus` limit for glues is reached.
+
+* \y`\tracingonline=0`,
+  \y`\tracingmacros=0`,
+  \y`\tracingstats=0`,
+  \y`\tracingparagraphs=0`,
+  \y`\tracingpages=0`,
+  \y`\tracingoutput=0`,
+  \y`\tracinglostchars=1`,
+  \y`\tracingcommands=0`,
+  \y`\tracingrestores=0`,
+  \y`\tracingscantokens=0`, 
+  \y`\tracingifs=0`,
+  \y`\tracinggroups=0`, 
+  \y`\tracingassigns=0`. 
+  If these registers have positive values then \TeX/ reports details about
+  the processing of build-in
+  algorithms to the log file. If \i tracingonline `\tracingonline>0` then the same
+  output is on the terminal.
+
+* \y`\showboxbreadth=5`,
+  \y`\showboxdepth=3`,
+  \y`\errorcontextlines=5`.
+  The amount of information when boxes are traced to the log file or an error is
+  reported.
+
+* \y`\language=0`. 
+  \TeX/ is able to load more hyphenation patters for more
+  languages. This register points to the index of currently used
+  hyphenation patterns. Zero means English.
+
+* \y`\lefthyphenmin=2`, \y`\righthyphenmin=3`. Maximal letters left or right of
+  hyphenated words.
+
+* \y`\defaulthyphenchar=`\code{`}`\-`. This character is used when words are
+  hyphenated.
+
+* \y`\globaldefs=0`. If it is positive then all settings are global.
+
+* \y`\hangafter=1`,
+  \y`\hangindent=0pt`.
+  If \x`\hangindent` is positive, then after \x`\hangafter` lines all following
+  lines are indented. Negative/positive values of \x`\hangindent` or \x`\hangafter` does
+  indentation from left or right and from the top or bottom of the paragraph.
+  The \x`\hangindet` is set to 0 after each paragraph.
+
+* \y`\mag=1000`. Magnification factor of all used dimensions.
+  The value 1000 means 1:1.
+
+* \y`\escapechar=`\code{`}`\\` 
+  use this character in the `\string` primitive.
+
+* \y`\newlinechar=-1`. If positive, this character is interpreted as the end of
+  the line when printing to the log or by \x`\write` primitive command.
+
+* \y`\endlinechar=`\code{`}`^^M`. 
+  This character is appended to the end of each input
+  line. Tokenizer converts it (Ctrl-M character) to the space token.
+
+* \y`\time=now`,
+  \y`\day=now`,
+  \y`\month=now`,
+  \y`\year=now`. The values about current time/date are set here when \TeX/
+  starts to process the document. The \x`\time`
+  counts minutes after midnight.
+
+* \y`\overfullrule=5pt`. A rectangle to this width is appended after each
+  \ii overfull/box overfull `\hbox`.
+
+* \y`\mathsurround=0pt` is space inserted around formula from internal math mode.
+
+* \y`\abovedisplayskip=12pt plus3pt minus9pt`,
+  \y`\abovedisplayshortskip=0pt plus3pt`,
+  \y`\belowdisplayskip=12pt plus3pt minus9pt`,
+  \y`\belowdisplayshortskip=7pt plus3pt minus 4pt`.
+  These spaces are inserted above and below the formula generated in math display
+  mode.
+
+* \y`\tabskip=0pt` is used by `\halign` primitive command for creating tables.
+
+* \y`\output={\plainoutput}`, \y`\everypar={}`, \y`\everymath={}`
+  \y`\everydisplay={}`, 
+  \y`\everyhbox={}`
+  \y`\everyvbox={}`
+  \y`\everycr={}`,
+  \y`\everyjob={}`.
+  These tokens lists are processed when algorithms of \TeX/ reach a corresponding
+  situation: opens output routine, paragraph, internal math mode, display
+  math mode, \x`\vbox`, \x`\hbox`. Or it is at the end of the line in the table or it
+  starts the job.  
+\enditems
+
+\sec[expand] Expandable primitive commands
+
+Notes about notation in this and the following sections. 
+If the documented command is from e\TeX{} extension 
+(i.\,e.\ implemented in pdf\TeX, Xe\TeX/
+and lua\TeX) then one * is prefixed. If it is from pdf\TeX/ extension
+(implemented in Xe\TeX/ and lua\TeX/ too) then two ** are prefixed. 
+If it is~a~lua\TeX/ only command then three *** are prefixed.
+
+\begitems
+* \i string `\string|<control sequence>;` expand to an \x`\escapechar` (if positive)
+  followed by the name of the control sequence. All characters of the output
+  are \"other characters\c{12}", only spaces (if exist)
+  are kept as space tokens {\char9251}\c{10}.  
+
+* \*\*\*\i csstring `\csstring|<control sequence>;` works like`\string` but without
+  `\escapechar`.
+
+* \*\i detokenize `\detokenize/<expandafters>.{|<text>};` re-tokenizes all tokens in the text.
+  Control sequences used in <text> are re-tokenized like `\string` primitive, spaces
+  are tokens {\char9251}\c{10} and all other tokens are set as \"other
+  characters\c{12}".
+
+* \i the `\the|<register>;` expands to the value of the register. Examples were in
+  previous section. The output is tokenized like the output of `\detokenize`.
+  The exception is `\the|<tokens register>;`: the output is the value
+  of the `<tokens register>` without re-tokenizing and expand processor 
+  does not expand this output in `\edef`\, `\write`, `\message` etc. arguments.
+
+* \i scantoken `\scantokens/<expandafters>.{|<text>};` re-tokenizes <text> using actual
+  tokenizer setting. The behavior is the same as to write `<text>` to a 
+  virtual file and reading this file immediately. 
+
+* \*\*\*\i scantextokens `\scantextokens/<expandafters>.{|<text>};` is the same as
+  `\scantokens` but removes problems with end-of-virtual-file.
+
+* \i meaning `\meaning|<token>;` expands to the meaning of the `<token>`. The text is
+  tokenized like in the `\detokenze` output. 
+
+* \i csname \i endcsname `\csname<text>\endcsname` creates a control sequence with name <text>.
+  If it is not defined, then it gets the `\relax` meaning.
+  For example `\csname TeX\endcsname` is the same as `\TeX`.
+  The `<text>` must be expandable to characters only. Non-expandable
+  control sequences (a primitive command at the main processor level, a register,
+  a character constant, a font selector) are disallowed here. \TeX/ reports the
+  error `missing \encsname` when this rule isn't compliant.
+
+  Example: `\csname foo:\the\mynumber\endcsname` expands to control sequence
+  `\foo:42` if the `\mynumber` is a register with the value 42. 
+  Another example: macro programmer should implement key/value dictionary 
+  using this primitive: 
+\begtt
+\def\keyval #1 #2 {\expandafter\def\csname dict:#1\endcsnme{#2}}
+\def\value #1 {\csname dict:#1\endcsname} 
+\kyval Peter 21 % key=Peter, value=21, saved to the dictionary
+                % it does \def\dict:Peter{21}
+\value Peter    % expands to \dict:Peter and it expands to 21. 
+\endtt
+
+* \i expandafter `\expandafer|<token 1><token 2>;` does transformation
+  `<token 1><expanded token2>`. The token processor will expand `<token 1>`
+  after such transformation. The `<expanded token2>` is only the first level of
+  expansion. For example, a macro is transformed to its `<replacement text>`
+  but without expansion of `<replacement text>` at this time.
+  Or `\csname...\endcsname` pair creates a control sequence but does not
+  expand it at this time.
+
+  If `<token 2>` is not expandable then \x`\expandafter` silently does
+  nothing.
+
+  The example above (the `\keyval` macro) shows usage of \x`\expandafter`.
+  We need not define `\csname` by `\def`, we want to define 
+  a `\dict:key`. The \x`\expandafter` helps here.
+
+  The `<token 2>` should be another \x`\expandafter`. We can see
+  \x`\expandafter` chains in many macro files. For example
+  `\expandafter A\expandafter B\expandafter CD` is processed as
+  `ABC<expanded D>`.  
+
+  The `/<expandafters>.{|<text>};` syntax rule enables to prepare `|<text>;` using
+  `\expandafter`(s). For example \i detokenize `\detokenize{\macro}` expands to
+  `\`\c{12}`m`\c{12}`a`\c{12}`c`\c{12}`r`\c{12}`o`\c{12}. But if you need to detokenize
+  the `<replacement text>` of the `\macro` then use
+  `\detokenize\expandafter{\macro}`. Not only `\expandafter`s should be
+  here. Expand processor does full expansion here until the opening brace 
+  `{`\c{1} is found.
+
+* \i if \i else \i fi The general rule for all `\if*` commands is 
+  `<if condition><true text>/\else<false text>.\fi`. 
+  The `<if condition>` is evaluated and `<true text>` or `<false text>`
+  is skipped or processed depending on the result of `<if condition>`.
+  When the expand processor is skipping the text due to `\if*` command then it
+  expands nothing in the skipped text. But it is noticing all control
+  sequences with meaning `\if*`, `\else` and `\fi` during skipping in order 
+  to skip correctly all nested `\if*.../\else....\fi` constructions.
+
+  The following `<if condition>`s are possible:
+  \_printitem={$\circ$\enspace}
+
+* \i if `\if<token 1><token 2>` is true if
+  \begitems \removelastskip \style a 
+  * both tokens are characters with the same Unicode (or ASCII code in classical \TeX) or
+  * both tokens are control sequences 
+   (with arbitrary meaning but not \"the character") or
+  * one token is a character, second is a control sequence equal to the character (by `\let`) or
+  * both tokens are control sequences, their meaning (set by `\let`) is the same character code.
+  \enditems
+  \removelastskip
+  \noindent Example: you can say `\let\test=a` then `\if\test a` returns true.
+
+* \i ifx `\ifx|<token 1><token 2>;` is true if the meanings of `<token 1>`
+   and `<token 2>` are the same. 
+
+* \i ifnum `\ifnum<number 1><relation><number 2>`. The `<relation>` could be
+  \code{<} or \code{=} or \code{>}. It returns true if the comparison of two
+  numbers is true.
+
+* \i ifodd `\ifodd<number>` returns true if the `<number>` is odd.
+
+* \i ifdim `\ifdim<dimen><relation><dimen>` The `<relation>` could be
+  \code{<} or \code{=} or \code{>}. It returns true if the comparison of two
+  dimensions is true.
+
+* \x`\iftrue` returns constantly true, \x`\iffalse` returns constanty false.
+
+* \x`\ifhmode`, \x`\ifvmode`, \x`\ifmmode` -- 
+   true if the current mode is horizontal, vertical, math.
+
+* \x`\ifinner` returns true if the current mode is internal vertical, internal
+  horizontal or internal math mode.
+
+* \i ifhbox `\ifhbox<box number>`, 
+  \i ifvbox `\ifvbox<box number>`, \i ifvoid `\ifvoid<box number>`
+  returns true if the specified `<box number>` represents \x`\hbox`, \x`\vbox`, void box
+  respectively.
+
+* \i ifcat `\ifcat<token 1><token 2>` is true if catogory codes of `<token 1>`
+  and `<token 2>` equals.
+
+* \i ifeof `\ifeof<file number>` is true, of the file attached to the `<file number>`
+  by \x`\openin` primitive does not exist or the end of file was reached by
+  the \x`\read` primitive.
+  \_printitem{$\bullet$\enspace}
+
+* \*\i unless `\unless<if condition>` negates the result of `<if condition>` before
+   skipping or processing the following text.
+
+* \i ifcase `\ifcase<number><case 0>\or<case 1>\or<case 2>...\or<case n>/\else<else text>.\fi`. 
+  It processes the branch given by `<number>`. It processes `<else text>`
+  (or nothing if no `<else text>` is declared) when a branch with given
+  `<number>` does not exist.
+
+* \i noexpand `\noexpand|<token>;`. 
+   The expand processor does not expand the `<token>` if it is expanding the
+   text in `\edef`, `\write`, `\message` or similar lists. 
+
+* \*\i unexpanded `\unexpanded/<expandafters>.{|<text>};` returns `<text>` and applies 
+  `\noexpand` to all tokens in the `<text>`. 
+
+* \*\i numexpr `\numexpr<num. expression>`, \i dimexpr \*`\dimexpr<dimen expression>`.
+  Documented in the `<dimen>` and `<number>` syntax rules in the section~\ref[reg].
+
+* \i number `\number<number>`, \i romannumeral `\romannumeral<number>` prints <number> in decimal
+  digits or as a roman numeral (with lowercase letters).
+
+* \x`\topmark` (last from previous page), 
+  \x`\firstmark` (first on current page), 
+  \x`\botmark` (last on current page). They expand to the corresponding
+  \x`\mark` included in the current or previous page-box. 
+  Usable for implementing running headers in the output routine.
+
+* \i fontname `\fontname<font selector>` expands to the file name \*\*\*(or font name) of
+  the font given by its `<font selector>`. The `\fontname\font` expands to
+  the file name of the current font. 
+
+* \x`\jobname` expands to the name of the main file of this document (without
+  extension `.tex`).
+
+* \i input `\input<file name><space>` (classical \TeX) or \ `\input"<file name>"` \
+  or \ `\input{<file name>}` \ opens the given
+  `<file name>` and starts to read input from it. If the `<file name>` 
+  doesn't exist then \TeX/ tries to open again \,{\def\,{\kern-1pt}`<file name>.tex`}. 
+  If it doesn't exist even this, \TeX/ reports an error.
+  The alternative syntax  with `"..."`  or `{...}` allows having spaces in the file
+  names.
+
+* \i endinput `\endinput`. The current line is the last line of the inputted file. File
+  is closed and reading continues from the place where `\input` of this file 
+  was started. `\endinput` done in the main file causes future reading from the 
+  terminal and headache of users.
+
+* \*\*\*\i directlua `\directlua {<text>}` runs a Lua script given in `<text>`.
+
+\enditems
+
+
+\sec[main] Primitive commands at the main processor level
+
+{\bf Commands used for declaration control sequences}
+
+\begitems
+* \x`\def` \x`\edef` \x`\gdef` \x`\xdef` were documented in the section~\ref[def].
+
+* \x`\long` is prefix, it can be used before `\def` `\edef` `\gdef` `\xdef`.
+  The declared macro accepts the control sequence `\par` in its parameters.
+
+* \*\x`\private` is prefix, it can be used before `\def` `\edef` `\gdef`, `\xdef`.
+  The declared macro is not expanded by the expand processor in \x`\write`,
+  \x`\message`, \x`\edef` etc. parameters.
+
+* \x`\outer` is prefix, can be used before `\def` `\edef` `\gdef`, `\xdef`.
+  The declared macro must be used only when the main processor is in the context 
+  {\em do something} or \TeX/ reports an error. 
+
+* \x`\global` is a prefix, it can be used before each assignment (commands
+  from this subsection and `<register>=<value>` settings). The
+  assignment is global regardless of the current group.
+
+* \i chardef `\chardef|<control sequence>;=<number>`, 
+  \i mathchardef `\mathchardef|<control sequence>;=<number>`
+  \ declares a constant <number>. When the main processor is in the context
+  \"do something" and it gets a \x`\chardef`-ed control sequence, it prints
+  the character with Unicode (ASCII code) <number> to the typesetting output. 
+  If it gets a \x`\mathchardef`-ed control sequence, it prints a math object (it works
+  only in math mode, not documented here).
+
+* \i countdef `\countdef|<control sequence>;=<number>` declares `<control sequence>` as
+  an equivalent to the `\count<number>` which is a register of counter type. 
+  The `<number>` here means an address in the
+  array of registers of counter type. The `\count0` is reserved for the page
+  number. The macro programmer uses rarely direct addresses (1 to 9), more
+  common is using the allocation macro `\newcount|<control sequence>;`.
+ 
+* \x`\dimendef`, \x`\skipdef`, \x`\muskipdef`, \x`\toksdef` followed by
+  `|<control sequence>;=<number>` 
+  declare analogically 
+  equivalents to \i dimen `\dimen<number>`, \i skip `\skip<number>`, 
+  \i muskip `\muskip<number>` and \i toks `\toks<number>`. 
+  Usage of allocation macros \x`\newdimen`, \x`\newskip`, \x`\newmuskip`,
+  \x`\newtoks` are preferred.
+
+* \i font `\font|<font selector>;=<file name><space>/<size specification>.` declares
+  `<font selector>` of a font implemented in the `<file name>.tfm`. The 
+  `<size specifiaction>` can be `at<dimen>` or `scaled<factor>`.
+  The `<factor>` equal to {\tt 1000} means 1:1.
+  New syntax (supported by Unicode engines) is
+
+\begtt \catcode`\|=13 \catcode`\/=13 \catcode`\<=13 \Blue
+\font|<font selector>;="<font name>/:<font features>." /<size specification>.  % or 
+\font|<font selector>;="[<font file>]/:<font features>." /<size specification>.  
+\endtt
+%
+  The `<font file>` is file name without `.otf` nor `.ttf` extension.
+  The `<font features>` are font features prefixed by `+` or `-` and
+  separated by semicolon. The {\let\,=\relax `otfinfo -f <file name>.otf`} command 
+  (on command line) can list them. 
+  Lua\TeX/ supports alternative syntax: `{...}` instead of `"..."`.
+  Example: `\font\test={[texgyretermes-regular]:+onum;-liga} at12pt`. 
+
+* \i let `\let|<control sequence>=<token>;` sets to the `<control sequence>`
+  the same meaning as `<token>` has. The `<token>` can be whatever, a
+  character or a control sequence.
+
+* \i futurelet `\futurelet|<control sequence><token 1><token 2>;` works in two steps.
+  In the first step it does `\let|<control sequence>=<token 2>;` and in the
+  second step `<token 1><token 2>` is processed with activated token
+  processor. Typically `<token 1>` is a macro that needs to know the next token.
+\enditems
+
+\noindent {\bf Commands for box manipulation}
+
+\begitems
+* \i hbox `\hbox{<cmds>}` or `\hbox to<dimen>{<cmds>}` or `\hbox spread<dimen>{<cmds>}`
+  creates a box. The material inside this box is a `<horizontal list>`
+  generated by `<cmds>` in the horizontal mode in a group. The width of the box is natural width
+  of the `<horizontal list>` or `<dimen>` given by \ii to `to<dimen>` parameter or
+  it is spread by `<dimen>` given by \ii spread `spread<dimen>`
+  parameter. The height of the box is the maximum of heights of all elements in
+  the `<horizontal list>`. The depth of the box is maximum of depths of all
+  such elements. These elements are put to the common baseline (exceptions
+  can be given by \x`\lower` or \x`\raise` commands). 
+
+* \i vbox `\vbox{<cmds>}` or `\vbox to<dimen>{<cmds>}` 
+  or `\vbox spread<dimen>{<cmds>}`
+  creates a box. The material inside this box is a `<vertical list>`
+  generated by `<cmds>` in the vertical mode in a group. The height of the
+  box is the natural height of the `<vertical list>` (eventually modified by values
+  from \ii to `to` or \ii spread `spread` parameters) without the depth of the last
+  element. The depth of the last element is set as the depth of the box. 
+  The width of the box is the maximum of widths of elemens in the `<vertical list>`.
+  All elements are put to the
+  common left margin of the box (exceptions can be given by \x`\moveleft` or
+  \x`\moveright` commands).
+
+* \i vtop `\vtop{<cmds>}` (with optional `to` or `spread`
+  parameters) is equal as `\vbox`,
+  but the baseline of the resulting box goes through the baseline of the first element in
+  the `<vertical list>` (note that `\vbox` has its baseline equal to the
+  the baseline of the last element inside).
+
+* \i vcenter `\vcenter{<cmds>}` (with optional `to` or `spread`
+  parameters) is equal to
+  `\vbox`, but its \ii math/axis {\em math axis}\fnote
+  {Math axis is a horizontal line which goes through centers of + and $-$
+  symbols. Its distance from baseline is declared in the math font metrics.}
+  is exactly in the middle of the box. So its baseline is appropriately shifted.
+  The `\vcenter` can be used only in math modes but given `<cmds>` are
+  processed in vertical mode.
+
+* \i lower `\lower<dimen><box>`, \i raise `\raise<dimen><box>` move the `<box>` up or down by
+  the `<dimen>` in horizontal mode. 
+  \i moveleft `\moveleft<dimen><box>`, 
+  \i moveright `\moveright<dimen><box>` move the `<box>` by
+  the `<dimen>` in vertical mode.
+
+* \i setbox `\setbox<box number>=<box>`. \TeX/ has a set of 
+  \ii box/register {\em box registers} addressed by `<box number>` and
+  accessed via \i box `\box<box number>` or alternatives described below.
+  The `\setbox` command saves given `<box>` to the register addressed by
+  `<box-number>`. 
+
+  Macro programmers use only 0 to 9 `<box numbers>` directly. Other
+  addresses to box registers should be allocated by 
+  the \i newbox `\newbox|<control sequence>;` macro. The `|<control sequence>;`
+  is equivalent to a `<box number>`, not to the box register itself.
+
+  The `\setbox` command does an assignment, so \x`\global` prefix is needed
+  if you want to use the saved box outside the current group.
+
+* \i box `\box<box number>` returns the box from `<box number>` box
+  register. Example: you can do `\setbox0=\hbox{abc}`. This `\hbox` is
+  not printed but saved to the register 0. At different place you can use
+  `\box0` which prints `\hbox{abc}` or you can do
+  `\setbox0=\hbox{cde\box0}` which saves the `\hbox{cde\hbox{abc}}`
+  to the register~0.
+
+* \i copy `\copy<box number>` returns the box from
+  `<box number>` box register and keeps the same box in this box register.
+  Note that the \i box `\box<box number>` returns the
+  box and empties the register `<box number>` immediately. If you don't want
+  to empty the register, use `\copy`. 
+
+* \i wd `\wd<box number>`, `\ht<box number>`, `\dp<box number>`. You can
+  measure or use the width, height and depth of a box saved in a register addressed
+  by `<box nuber>`. Examples `\mydimen=\ht0`, `\hbox to\wd0{...}`. 
+  You can re-set the dimensions of a box saved in a register addressed by 
+  `<box number>`. For example \i setbox `\setbox0=\hbox{abc}` `\wd0=0pt` `\box0`
+  gives the same result as \i hbox `\hbox to0pt{abc}` but without the warning about
+  \ii overfull/box overfull `\hbox`.
+
+* \i unhbox `\unhbox<box number>`, \kern-.4pt\i unvbox `\unvbox<box number>`, 
+  \kern-.4pt\i unhcopy `\unhcopy<box number>`, \kern-.4pt\i unvcopy `\unvcopy<box number>`
+  do the same work as `\box` or `\copy` but they don't return the whole
+  box but only it contents, i.\,e.~a horizontal or vertical material.
+  Example: try to do \i setbox `\setbox0=\hbox{abc}` and later 
+  `\setbox0=\hbox{cde\unhbox0}` saves
+  the \i hbox `\hbox{cdeabc}` to the box register~0. 
+
+  The \x`\unhbox` and \x`\unhcopy` commands return the \x`\hbox` contents and 
+  \x`\unvbox`, \x`\unvcopy` commands return the \x`\vbox` contents. If incompatible
+  contents is saved then \TeX/ reports an error. You can test the type of
+  saved contents by \x`\ifhbox` or \x`\ifvbox`.
+
+* \i vsplit `\vsplit<box number> to<dimen>` does a column break. 
+  The `<vertical material>` saved in the box `<box number>` is broken
+  to a first part of `<dimen>` height and the rest remains in the
+  box `<box number>`. The broken part is completed as `\vbox` which is the
+  result of this operation. For example, you can say `\newbox\column`
+  `\setbox\column=\vbox{...}` and later 
+  `\setbox0=\vsplit\column to5cm`. The `\box0` is `\vbox` with first 5cm of
+  saved material.
+
+* \x`\lastbox` returns the last box in the current vertical or horizontal
+  material and removes it. 
+\enditems
+
+\noindent {\bf Commands for rules (lines in the typesetting output) and patterns}
+
+\begitems
+* \x`\hrule` creates a horizontal line in current vertical
+  list. If it is used in a horizontal mode, it finishes the paragraph by
+  \x`\par` first. 
+  `\hrule /width<dimen>. /height<dimen>. /depth<dimen>.` creates (in
+  general, with given parameters) a full
+  rectangle (something like a box, but it isn't treated as the box) with given
+  dimensions. Default values are: \"width"=width of outer `\vbox`,
+  \"height"=0.4\,pt, \"depth"=0\,pt.
+  {\emergencystretch=2em\par}
+
+* \x`\vrule` creates a vertical line in current horizontal
+  list. If it is used in a vertical mode, it opens the horizontal mode
+  first.
+  `\vrule /width<dimen>. /height<dimen>. /depth<dimen>.` creates (in
+  general, with given parameters) a full rectangle with given
+  dimensions. Default values are: \"width"=0.4\,pt,
+  \"height"=height of outer `\hbox`, \"depth"=depth of outer `\hbox`.
+  {\emergencystretch=2em\par}
+
+  The optional parameters of `\hrule` and `\vrule` could be specified in
+  arbitrary order and they can be specified more than once. In such a case,
+  the rule \"last wins" is applied.
+
+* \i leaders `\leaders<rule><glue>` crates a glue 
+  (maybe shrinkable or stretchable) filled by a full rectangle.
+  The `<rule>` is \x`\vrule` or \x`\hrule` (maybe with its optional parameters).
+  If the `<glue>` is specified by \x`\hskip` command
+  (maybe with its optional parameters)
+  or by its alternatives \x`\hss`, \x`\hfil`, \x`\hfill`, then the resulting glue
+  is horizontal (can be used only in horizontal mode) and its dimensions
+  are: width derived from `<glue>`, height plus depth derived from `<rule>`.
+  If the `<glue>` is specified by \x`\vskip` command
+  (maybe with its optional parameters)
+  or by its alternatives \x`\vss`, \x`\vfil`, \x`\vfill`, then the resulting glue
+  is vertical (can be used only in vertical mode) and its dimensions
+  are: height derived from `<glue>`, width derived from `<rule>`, depth is zero.
+
+* \i leaders `\leaders<box><glue>` creates a vertical or horizontal glue
+  filled by a pattern of repeated `<box>`. The positions of boxes are
+  calculated from the boundaries of the outer box. It is used for dots patterns 
+  in the table of contents.
+  \i cleaders `\cleaders<box><glue>` does the same, but the pattern of boxes is
+  centered in the space derived by the <glue>. Spaces between boxes are not~inserted. 
+  \i xleaders `\xleadres<box><glue>` does the same, but the spaces between
+  boxes are inserted equally.
+\enditems
+
+\noindent {\bf More commands for creating something in typesetting output}
+
+\begitems
+* \x`\par` closes horizontal mode and finalizes a paragraph.
+* \x`\indent`, \x`\noindent`. They leave the vertical mode and open paragraph
+  with/without paragraph indentation. If horizontal mode is current then 
+  `\indent` inserts an empty box of `\parindent` width, `\noindent` does
+  nothing.
+* \x`\hskip`, \x`\vskip`. They insert a horizontal/vertical glue. Documented in
+  the section~\ref[boxes].
+* \x`\hfil`, \x`\hfill`, \x`\hss`, \x`\vfil`, \x`\vfill`, \x`\vss` are alternatives
+  of \x`\hskip`, \x`\vskip`. 
+* \i kern `\kern<dimen>` puts nonbreakable horizontal/vertical space 
+  depending on the current mode.
+* \i penalty `\penalty<number>` puts the penalty `<number>` to the current
+  horizontal/vertical list.
+* \i char `\char<number>` prints the character with code 
+  \kern-2pt`<number>`\kern-2pt. The
+  \"character itself" does the same.
+* \i accent `\accent<number><character>` places an accent with code
+  `<number>` above the `<character>`.
+* \ii -space `\`{\tt\char9251} is \ii control/space control space. In
+  horizontal mode, it inserts the space glue (like normal space but without
+  modification by the \x`\spacefactor`). In vertical mode, it opens horizontal
+  mode and puts the space. Note that normal space does nothing in vertical
+  mode.
+* \i discretionary `\discretionary{<pre break>}{<post break>}{<no break>}` 
+  works in horizontal mode. It prints `<no break>`
+  in normal cases but if there is a line break then `<pre break>`
+  is used before and `<post break>` after the breaking point.
+  German Zucker/Zuk-ker (sugar) can be implemented by 
+  `Zu\discretionary{k-}{k}{ck}er`.
+* \ii -hyphen `\-` is equal to \i hyphenchar 
+  `\discretionary{\char\hyphenchar<font>}{}{}`.
+  The `\hyphenchar<font>` is used as a hyphenation character. It is set
+  to \x`\defaulthyphenchar` value when the font is loaded, but it can be changed.
+* \ii -italiccorr \code{\\/} does an
+  \ii italics/correction italics correction. 
+  It puts a little space if the last character is slanted.
+* \x`\unpenalty`, \x`\unskip` removes last penalty / last glue
+  from the current horizontal/vertical list.
+* \i vadjust `\vadjust{<cmds>}`. It works in horizontal mode. The `<cmds>`
+  must create a `<vertcal list>` and `\vadjust` saves a pointer to this list
+  into the current horizontal list. When `\par` creates lines of the paragraph
+  and distributes them to a vertical list, each line with the pointer from
+  `\vadjust` has the corresponding `<vertical list>` immediately appended 
+  after this line.
+* `\insert<number>{<cmds>}`. The `<cmds>` create a `<verical list>` and
+  `\insert` saves a pointer to such `<vertical list>` into the current list.
+  The output routine can work with such `<vertical list>`\kern-2pts. The footnotes or
+  \ii floating~object {\em floating objects} (tables, figures) 
+  are implemented by the `\insert` primitive.  
+* \i halign `\halign{<declaration>\cr<row 1>\cr<row 2>\cr...\cr<row n>\cr}`
+  creates a table of boxes in vertical mode. The `<declaration>` declares
+  one or more column patterns separated by `&`\c4. The rows use the same
+  character to separate the items of the table in each row. The `\halign`
+  works in two passes. First it saves all items to boxes and the second pass
+  performs `\hbox to`~$\Blue w$ for each saved items, where $\Blue w$ 
+  is the maximal width of items in each actual column. 
+
+  Detailed documentation of `\halign` is out of scope of this manual.
+  Only one example follows: the macro `\putabove` puts `#1` above `#2`
+  centered. The width of resulting box is equal to max of widths of these 
+  two parameters. The `<declaration>` `\hfil##\hfil` means that the items
+  will be centered:
+  `\def\putabove#1#2{\vbox{\halign{\hfil##\hfil\cr#1\cr#2\cr}}}`.
+
+* \x`\valign` does the same as `\halign` but rows $\leftrightarrow$ columns.
+  It is not used commonly.
+
+* \x`\cr`, \x`\crcr`, \x`\span`, \x`\omit`, \i noalign `\noalign{<cmds>}`
+  are primitives used by `\halign` and `\valign`. 
+
+\enditems
+
+\noindent {\bf Commands for registers calculation}
+
+\begitems
+* \i advance `\advance<register>/by.<value>` does (formally)
+  `<register>=<register>+<value>`. The `<register>` is counter type or dimen
+  type. The `<value>` is `<number>` or `<dimen>` (depending on type of the
+  `<register>`).
+* \i multiply `\multiply<register>/by.<number>` does
+  `<register>=<register>*<number>`.
+* \i divide `\divide<register>/by.<number>` does
+  `<register>=<register>`\code{/}`<number>`. If the `<register>` is 
+  number type then the result is truncated.
+* See \*`\numexpr` and \*`\dimexpr`, expandable primitives documented in the
+  sections~\ref[reg] and~\ref[expand].
+\enditems
+
+\noindent {\bf Internal codes}
+
+\begitems
+* \i catcode `\catcode<number>` is category code of the character with
+  `<number>` code. Used by tokenizer.
+* \i lccode `\lccode<number>` is lower case alternative to the
+  `\char<number>`. If it is zero then lower case alternative doesn't exist
+  (for example for punctuation). Used by `\lowercase` primitive and when
+  breaking points are calculated from hyphenation patterns.
+* \i uccode `\uccode<number>` is upper case alternative to the
+  `\char<number>`. If it is zero, then the upper case alternative doesn't exist.
+  Used by `\uppercase` primitive.
+* \i lowercase \i uppercase `\lowercase/<expandafters>.{|<text>};`, \
+  `\uppercase/<expandafters>.{|<text>};` transform `|<text>;` to
+  lowercase / uppercase using current `\lccode` or `\uccode` values.
+  Returns transformed `|<text>;` where catcodes of tokens and 
+  tokens of type `<control sequence>` are unchanged.
+* \i sfcode `\sfcode<number>` is spacefactor code of the `\char<number>`.
+  The `\spacefactor` register keeps (roughly speking) 
+  the `\sfcode` of the last printed character. The glue between words
+  is modified (roughly speaking) by this `\spacefactor`. The value
+  1000 means factor 1:1 (no modification is done). It is used for enlarging spaces
+  after periods and other punctuation in English texts.\fnote{
+  This feature is not compliant with another typographical traditions, so
+  `\frenchspacing` macro which sets all `\sfcodes` to 1000 is used very often.}
+  {\emergencystretch=2em\par}
+\enditems
+
+\noindent {\bf Commands for reading or writing text files}
+
+\begitems
+* Note, that main input stream is controlled by `\input` and `\endinput`
+  expandable primitive commands documented in the section~\ref[expand].
+
+* \i openin `\openin<file num>=<file name><space>`
+  (or `\openin<file num>={<file name>}`) opens the
+  file `<file name>` for reading and creates a file descriptor
+  connected to the `<file num>`\fnote
+  {Note that `<file num>` is an addresses to the file descriptor. Macro
+  programmers don't use these addresses directly but by 
+  \i newread `\newread|<control sequence>;` and 
+  \i newwrite `\newwrite|<control sequence>;` allocation macros.}. 
+  If the file doesn't exist nothing happens but
+  macro programmer can test this case by `\ifeof<file num>`.
+
+* \i read `\read<file num>to|<control sequence>;` does
+  `\def|<control sequence>{<replacement text>};` where
+  the `<replacement text>` is tokenized next line from the file declared by
+  `\openin` as `<file num>`.
+
+* \i openout `\openout<file num>=<file name><space>`
+  (or `\openout<file num>="<file name>"`) \  opens the `<file name>` for
+  writing and creates a file descriptor connected to `<file num>`.
+  If the file does exist, then its contents are removed.
+  {\emergencystretch=2em\par}
+
+* \i write `\write<file num>|{<text>};` writes a line of `<text>`
+  to the file declared by `\openout` as `<file num>`. But this isn't done
+  immediately. \TeX/ does not know the value of the current page
+  when the `\write` command is processed because
+  the paragraph building and page building algorithms are processed
+  asynchronously. But a macro programmer typically needs to save current 
+  page to the file in order to read it again and to create 
+  a Table of contents or an Index.
+
+  `\write<file num>|{<text>};` saves `|<text>;` into memory and puts a
+  pointer to this memory into the typesetting output. When the page is shipped
+  out (by output routine), then all such pointers from this page are
+  processed: the `<text>` is expanded at this time and its expansion
+  is saved to the file. If (for example) the `<text>` includes 
+  `\the\pageno` then it is expanded to the correct page number of this page. 
+
+* \i closein `\closein<file num>`, \i closeout `\closeout<file num>`
+  closes the open file. It is done automatically when \TeX/ terminates its
+  job.
+
+* \x`\immediate` is the prefix. It can be used before `\openout`, `\write` and
+  `\closeout` in order to do the desired action immediately (without waiting to
+  the output routine). 
+\enditems
+
+\noindent {\bf Others primitive commands}
+
+\begitems
+* \x`\relax` does nothing. Used for terminating uncomplete optional
+  parameters, for example.
+* \x`\begingroup` opens group, \x`\endgroup` closes group. 
+  The `{`\c1 and `}`\c2 does the same but moreover, they are syntactic
+  constructors for primitive commands and math lists (in math mode).
+  These two types of groups (declared by mentioned commands or
+  by mentioned characters) cannot be mixed, i.\,e.\ 
+  `\begingroup...}` gives an error. Plain \TeX/ declares
+  \x`\bgroup` and \x`\egroup` control sequences as an equivalent to
+  `{`\c1 and `}`\c2. They can be used instead  `{`\c1 and~`}`\c2 when we
+  need to open / close a group, to create a math list or when a box is constructed.
+  For example `\hbox\bgroup<text>\egroup` is syntactically correct.
+* \i aftergroup `\aftergroup|<token>;` saves the `|<token>;` and puts it
+  back in the input queue immediately after the current group is closed. 
+  Then expand processor expands it (if it is expandable). More
+  `\aftergroup`s in one group creates a queue of `|<token>;`s used after
+  the group is closed.
+* \i afterassignment `\afterassignment|<token>;` saves the `|<token>;`
+  and puts it back immediately after a following assignment (`<register>=<vaue>`,
+  `\def` etc.) is done.
+* \x`\lastskip`, \x`\lastpenalty` returns the value of last element in the
+  current horizontal or vertical list if it is glue / penalty. It returns
+  zero value if the element is not found as the last.
+* \x`\ignorespaces` ignores spaces in horizontal mode until
+  next primitive command occurs.
+* \i mark `\mark{<text>}` saves `<text>` to memory and puts a pointer to it in the
+  typesetting output. The `<text>` is used as expansion output of
+  \x`\firstmark`, \x`\topmark` and \x`\botmark` expansion primitives in output
+  the routine.
+* \i parshape `\parshape<number>`% 
+  {\def<#1>{\,$\langle\it#1\rangle$\,}%
+  `<I1><W1><I2><W2>...<In><Wn>`
+  enables to set arbitrary shape of the paragraph. The `<number>` 
+  declares the amount of data: the `<number>` pairs of `<dimen>`s follow.
+  The $i$-th line of the paragraph is shifted by `<Ii>` to right and
+  its width is `<Wi>`.} The `\parshape` data are re-set after
+  each paragraph to zero values (normal paragraph).
+* \i special `\special{<text>}` puts the message `<text>` to the typesetting output. It
+  behaves as a zero-dimension pointer to `<text>` and it can be read by
+  printer drivers. It is recommended to not use this old technology
+  when PDF output is created directly.
+* \i shipout `\shipout<box>` outputs the `<box>` as one page. Used in the 
+  \ii output/routine output routine.
+* \x`\end` completes the last page and terminates the job.
+* \x`\dump` dumps the memory image to the `\jobname.fmt` and terminates the job.
+* \i patterns `\patterns{<data>}` reads hyphenation patterns for current 
+  \x`\language`.
+* \i hyphenation `\hyphenation{<data>}` reads hyphenation exceptions for
+  current \x`\language`.
+* \i message `\message{<text>}` prints `<text>` on the terminal and to the 
+  log file.
+* \i errmessage `\errmessage{<text>}` behaves like `\message{<text>}` but
+  \TeX/ treats it as an error.
+* Job processing modes can be set by \x`\scrollmode` (don't pause at the
+  errors), \x`\nonstopmode` (don't pause at the errors and missing files),
+  \x`\batchmode` (\x`\nonstopmode` plus no output on the terminal). Default is
+  \x`\errorstopmode` (stop at errors).
+* \x`\inputlineno` includes the number of current line from current inputted
+  file.
+* \i show `\show|<control sequence>;`, \ \i showbox `\showbox<box number>`, 
+  \ \x`\showlists` \ and \ 
+  \i showthe `\showthe|<register>;` \ are tracing commands. \TeX/ prints desired result
+  on the terminal and to the log file and pauses.
+\enditems
+
+\noindent {\bf Commands specific for PDF output} 
+(available in pdf\TeX, \XeTeX/ and lua\TeX)
+
+\begitems
+* \i pdfliteral `\pdfliteral{<text>}` puts the `<text>` interpreted 
+  in a low level PDF language to the typesetting output. All PDF constructs
+  defined in the PDF specification are allowed. The dimensions of
+  the `\pdfliteral` object in the output are considered zero. So, if 
+  `<text>` moves the current typesetting point then the notion about its
+  position from \TeX/ point of view differs from the real position. 
+  A good practice is to close `<text>` to `q...Q` PDF commands.
+  The command `\pdfliteral` is typically 
+  used for generating graphics and for linear transformation.
+* \i pdfcolorstack `\pdfcolorstack<number><op>{<text>}` (where `<op>` is `push`
+  or `pop` or `set`) behaves like `\pdfliteral{<text>}` and it is used for
+  color switchers. For example when `<text>` is `1 0 0 rg` then red color is
+  selected. \TeX/ sets the colors stack at the top of each page to the
+  colors stack opened at the bottom of the previous page.
+* \i pdfximage 
+  `\pdfximage` `/height<dimen>.` `/depth<dimen>.` `/width<dimen>.` `/page<number>.{<file name>}`
+  loads the image from `<file name>` to the PDF output and returns the
+  number of such data object in the \x`\pdflastximage` register. Allowed
+  formats are PDF, JPG, PNG. The image is not drawn at this moment. Macro
+  programmer can save `\mypic=\pdflastximage` and draw the image by
+  \i pdfrefximage `\pdfrefximage\mypic` (maybe repeatedly). Data of 
+  the image are loaded to PDF output only once. 
+  The `\pdfximage` allows more parameters,
+  see pdf\TeX/ documentation.
+* \i pdfsetmatrix {\def<#1>{$\,\langle{\it#1}\rangle\,$}`\pdfsetmatrix {<a><b><c><b>}`
+  multiplies the current transformation matrix (used for linear
+  transformations) by `\matrix{<a>&<c>\cr <b>&<d>}`.}
+* \i pdfdest `\pdfdest name{<label>}<type>\relax` declares a destination of
+  a hyperlink. The `<label>` must match with the `<label>` used in 
+  `\pdfoutline` or `\pdfstartlink`. The `<type>` declares the behavior of
+  pdf viewer when the hyperlink is used. For example `xyz` means without changes
+  of the current zoom (if not specified). Other types should be `fit`, `fith`,
+  `fitv`, `fitb`. 
+* \i pdfstartlink 
+  `\pdfstartlink /height<dimen>. /depth<dimen>. /<attributes>. /goto name{<label>}.`
+  declares a begining of a hyperlink. A text (will be sensitive on mouse clik)
+  immediately follows and it is terminated by \x`\pdfendlink`. The height
+  and depth of sensitive area and the `<label>` used in `\pdfdest` are declared
+  here. More parameters are allowed, see the pdf\TeX/ documentation. 
+* \i pdfoutline `\pdfoutline /goto name{<label>}. /count<number>. {<text>}`
+  creates one item with `<text>` in PDF outlines. `<label>` must be used
+  somewhere by `\pdfdest name{<label>}`. The `<number>` is the number of
+  direct descentants in the outlines tree.
+* \i pdfinfo `\pdfinfo {<key>(<text>)}` saves to PDF the information which
+  can be listed by the command `pdfinfo <file>.pdf` on the command line for
+  example. More `<key>(<text>)` should be here. The `<key>` can be 
+  \code{/Author}, \code{/Title}, \code{/Subject},  
+  \code{/Kyewords}, \code{/Creator}, \code{/Producer},  
+  \code{/CreationDate}, \code{/ModDate}. The last two keywords need
+  a special format of the `<text>` value. All `<text>` values (including
+  `<text>` used in the `\pdfoutline`) must be ASCII encoded or they can use
+  a very special PDFunicode encoding.
+* \x`\pdfcatalog` enables to set a default behavior of PDF viewer when it starts.
+* \x`\pdfsavepos` saves an internal invisible point to the typesetting output. These
+  points are processed when the page is shipped out: the numeric registers
+  \x`\pdflastxps` and \x`\pdflastypos` get values about absolute position of
+  this invisible point (measured from the left upper corner of the page in `sp` units).
+  The macro programmer can follow `\pdfsavepos` by the `\write` command and save
+  these absolute positions to a text file which can be read in the next run
+  of \TeX/ in order to get these absolute positions by macros. 
+\enditems
+
+\noindent {\bf Microtypographical extensions}
+(available in pdf\TeX/, lua\TeX/ and not all of them in \XeTeX)
+
+\begitems
+* \i pdffontexpand 
+  `\pdffontexpand <font selector> <stretching> <shrinking> <step>`
+  declares a possibility to deform the characters from the font given by
+  `<font selector>`. This deformation is used when stretching or shrinking
+  paragraph lines or doing `\hbox to{...}` in general. I.\,e.\ not only glues are
+  stretchable and shrinkable. The numeric parameters are given in 1/1000
+  of the font size. `<stretching>` and `<shrinking>` are maximal allowed
+  values. The stretching nor shrinking are not applied continuously but by
+  given `<step>`.
+  To activate this feature you must to set the \x`\pdfadjustspacing`
+  numeric register to a positive value. 
+* \i rpcode \i lpcode 
+  `\rpcode <font selector><char. code>=<number>`,
+  `\lpcode <font selector><char. code>=<number>` allows to declare
+  hanging punctuation. Such punctuation is slightly moved to the right
+  margin (if `\rpcode` is declared and the character is at the right margin) 
+  or to left margin (for `\lpcode` analogically).
+  The `<number>` gives the amount of such moving in 1/1000 of the font size.
+  To activate this feature you must to set \x`\pdfprotrudechars` to
+  a positive value (2 or more means better algorithm).
+* \i letterspacefont
+  `\letterspacefont |<control sequence>; <font selector> <number>`
+  declares a new font selector `|<control sequence>;` as a font given by
+  the `<font selector>`. Additional space declared by `<number>` is added
+  between each two characters when the font is used. Tne `<number>` is 1/1000 of
+  font size. Unicoded fonts support analogical
+  `letterspace=<number>` font feature. 
+* The same syntax like `\rpcode` have the following commands:
+  \x`\knbscode` (added space after the character)
+  \x`\stbscode` (added stretchability of the glue after the character)
+  \x`\shbscode` (added shrinkability after the character)
+  \x`\knbccode` (added kenr before the character)
+  \x`\knaccode` (added kern after the character).
+  To activate this feature you must to set
+  \x`\pdfadjustinterwordglue` to a positive value.
+  This feature is supported by pdf\TeX/ only.
+\enditems
+
+\noindent {\bf Commands used in math mode}
+
+\begitems
+* \x`\displaystyle`, \x`\textstyle`, \x`\scriptstyle`, \x`\scriptscriptstye`
+  switches to specified style.
+* \x`\mathord`, \x`\mathop`, \x`\mathbin`, \x`\mathrel`, \x`\mathopen`, \x`\mathclose`,
+  \x`\mathpunct` followed by `{<math list>}` create an math object of given type.
+* \i over `{<nominator>\over<denominator>}` creates a fraction.
+  The primitive commands \x`\atop` (without fraction rule), 
+  \i above `\above<dimen>` (fraction rule with given thickness) should be used in the same
+  manner. The commands \x`\atopwithdelims`, \x`\overwithdelims`,
+  \x`\abovewithdelims` allow to specify brackets around the generalized
+  fraction.
+* \i left \i right `\left<delimiter><formula>\right<delimiter>` creates
+  a math `<formula>` and gives `<delimiter>`s around it with an appropriate size
+  (comparable with the size of the formula). The `<delimiter>`s are brackets
+  typically.
+* The exponents and scripts are typically at the right side of the previous
+  math object. But if this object is a \"big operator" (summation, integral)
+  then exponents and scripts are printed above and below this operator.
+  The commands \x`\limits`, \x`\nolimits`, \x`\displaylimits` 
+  used before exponents and scripts constructors 
+  (`^`\c7 and `_`\c8) declares an exception from this rule.
+* \i eqno `$$<formula>\eqno<mark>$$` puts the `<mark>` to the right
+  margin as `\llap{$<mark>$}`. Analogically 
+  \i leqno `$$<formula>\leqno<mark>$$` puts it to the left margin.
+\enditems
+
+\sec[plain] Summary of plain \TeX/ macros
+
+\noindent{\bf Allocators}
+
+\begitems
+* \x`\newcount`, \x`\newdimen`, \x`\newskip`, \x`\newmuskip`, \x`\newtoks`
+  folowed by a `|<control sequence>;` allocate a new register of given type
+  and set it as the `|<control sequence>;`. 
+  \x`\newbox`, \x`\newread`, \x`\newwrite` 
+  folowed by a `|<control sequence>;` allocate a new address to given data
+  (to a box register or to a file descriptor) and set is as the 
+  `|<control sequence>;`.
+  All these allocation macros are declared as `\outer` in plain \TeX/,
+  unfortunately. This brings problems when you need to use them in
+  skipped text or in macros (in `<replacement text>` for example).
+  Use `\csname newdimen\endcsname \yoursequence` in such cases. 
+* \i newif `\newif|<control sequence>;` sets the `|<control sequence>;`
+  as a boolean variable. It must begin with `if`; for
+  example `\newif\ifsomething`. Then you can set values by
+  `\somethingtrue` or `\somethingfalse` and you can use this variable by
+  `\ifsoemthing` which behaves like others `\if`\code{*} primitive commands.
+\enditems
+
+\noindent{\bf Vertical skips}
+
+\begitems
+* \x`\bigskip` does \x`\vskip` by one line, \x`\medskip` does `\vskip` by
+  one half of line and \x`\smallskip` does the vertical skip by one quarter of line.
+  The registers \x`\bigskipamount`, \x`\medskipamount` and \x`\smallskipamount`
+  are allocated for this purpose. 
+* \x`\nointerlineskip` ignores the \x`\baselineskip` rule 
+  %(see the section~\ref[reg]) 
+  for the following box in
+  the current vertical list. This box is appended immediately after
+  the previous box. 
+  \x`\offitnerlineskip` ignores the \x`\baselineskip` rule for all following
+  boxes until the current group is closed. 
+* All vertical glues at the top of the page inserted by \x`\vskip` are
+  ignored. Macro \x`\vglue` behaves like `\vskip` primitive command but its glue
+  is not ignored at the top of the page.
+
+* Sometimes we must switch off the \x`\baselineskip` rule (by \x`\offinterlineskip`
+  macro for example). This is common in the tables. But we need to keep the
+  baseline distances equal. Then the \x`\strut` can be inserted to each
+  line. It is an invisible box with zero width and with
+  height+depth=`\baselineskip`.
+
+* \x`\normalbaselines` sets the registers for vertical placement
+  \x`\baselineskip`, \x`\lineskip` and \x`\lineskiplimit` to default values given
+  by the format. User can set other values for a while and then he/she can use
+  `\normalbaselines`. 
+\enditems
+
+\noindent{\bf Penalties}
+
+\begitems
+* \x`\break` puts penalty -10000, so line/page break is forced here.
+  \x`\nobreak` puts penalty 10000, so line/page break is disabled here.
+  It should be preceded before a glue, which is \"protected" by this penalty.
+  \x`\allowbreak` puts penalty 0, it allows breaking similar as in normal space.
+* \x`\goodbreak` puts penalty -500 in vertical mode, this is \"recommended"
+  point for a page break.
+* \x`\filbreak` breaks the page only if it is \"almost full" or if a big object
+  (that doesn't fit the current page) follows. The bottom of such page is
+  filled by a vertical glue, i.\,e. the default typographical rule about equal
+  positions of all bottoms of common pages is broken here.
+* \x`\eject` puts penalty -10000 in vertical list, i.\,e.\ it breaks the page.
+\enditems
+
+\noindent{\bf Miscellaneous macros}\par\nobreak\medskip\nobreak
+
+\begitems
+* \i magstep `\magstep<number>` expands to a magnification factor $1.2^x$ where $x$ is
+  given `<number>`. This follows old typographical traditions that all sizes
+  (of fonts) are distinguished by factors 1, 1.2, 1.44, etc.
+  For example `\magstep2` expands to 1440, because $1.2^2=1.44$ 
+  and 1000 is factor 1:1 in \TeX/. The \x`\magstephalf` macro expands to
+  1095 which corresponds to $1.2^{(1/2)}$.
+* \x`\nonfrenchspacing` sets special space factor codes (bigger spaces after
+  periods, commas, semicolons etc.). This follows English typographical
+  traditions. \x`\frenchspacing` sets all space factors as 1:1 (usable for non
+  English texts). 
+* \x`\endgraf` is equavalent to \x`\par`, \x`\bgroup` and \x`\egroup` are
+  equivalents to `{`\c1 and `}`\c2.
+* \x`\space` expands to space, \x`\empty` is empty macro and \x`\null` is empty
+  \i hbox `\hbox{}`.
+* \x`\quad` is horizontal space 1\,em (size of the font), \x`\qquad` is
+  double `\quad`,
+  \x`\enspace` is kern 0.5\,em \x`\thinspace` is kern 1/6\,em and 
+  \x`\negthinspace` makes kern $-$1/6\,em.
+* \i loop \i repeat `\loop <body 1><if condition><body 2>\repeat`
+  repeats `<body 1>` and `<body 2>` in a loop until `<if condition>` returns
+  false. Then `<body 2>` is not processed and the loop is finished.
+* \x`\leavevmode` opens a paragraph like `\indent` but it does nothing if
+  the horizontal mode is opened already.
+
+* \i line `\line{<text>}` creates a box of line width (which is \x`\hsize`). 
+  \x`\leftline`, \x`\rightline`, \x`\centerline` do the same as 
+  `\line` but `<text>` is shifted left / right / is centered.
+
+* \i rlap `\rlap{<text>}` makes a box of zero size, the `<text>` is stuck
+  outright. \i llap `\llap{<text>}` does the same and the `<text>` is pushed
+  left. 
+
+* \x`\ialign` is equal to `\halign` but the values of registers used by
+  `\halign` are set to default.
+
+* \x`\hang` starts the paragraph where all lines (exception first) are
+  indented by `\parindent`.
+
+* \i textindent `\texindent{<mark>}` starts a paragraph with `\llap{<mark>}`. 
+
+* \i item `\item{<mark>}` starts the paragraph with `\hang` and with
+  `\llap{<mark>}`. Usable for item lists.
+  \i itemitem `\itemitem{<mark>}` can be used for the second level of items.
+
+* \x`\narrower` sets wider margins for paragraphs (`\parindent` is appended
+  to both sides), i.\,e.\ the paragraphs are narrower.
+
+* \x`\raggedright` sets the paragraph shape with the ragged right margin.
+  \x`\raggedbottom` sets the page-setting shape with the ragged bottoms.
+\enditems
+
+\noindent{\bf Floating objects}
+
+\begitems
+* \i footnote `\footnote{<mark>}{<text>}` creates a footnote with given
+  `<mark>` and `<text>`.
+
+* \i midinsert \i topinsert \i endinsert 
+  `\topinsert<object>\endinsert` create the <object> as a 
+  \ii floating~object {\em floating object}. It is printed on the top
+  of the current page or on the next page.
+  `\midinsert<object>\endinsert` does the same as `\topinsert` but it tries
+  if the `<object>` fits on the current page. If it is true then it is printed to
+  its current position, no floating object is created.
+\enditems
+
+
+\noindent{\bf Controlling of input, output}
+
+\begitems
+* \x`\obeyspaces` sets the space as normal, i.\,e. it deactivates special
+  treatment of spaces by tokenizer: more spaces will be more spaces and
+  spaces from the left line are not ignored.
+* \x`\obeylines` sets end of lines as `\par`. Each line in the input is
+  one paragraph in the output.
+
+* \x`\bye` finalizes the last page (or last pages if more floating objects
+  must be printed) and terminates the \TeX/ job. The \x`\end` primitive command
+  does the same but without worrying about floating objects.
+\enditems
+
+\noindent{\bf Macros used in math modes}
+
+\begitems
+* Spaces in math mode are 
+  \ii -comma `\,` (thin space), \ii -greater `\>` (medium space)
+  \ii -column `\;` (thick space, but still small), \ii -exclam `\!` (negative thin space).
+
+* \i choose `{<above>\choose<below>}` creates a combination number with
+  brackets around it.
+
+* \i sqrt `\sqrt{<math list>}` creates the square root symbol.
+  \i root `\root<n>\of{<math list>}` creates a general root symbol.
+
+* \i cases {\def<#1>{\,$\langle#1\rangle$\,}%
+  `\cases{<case 1>&<condition 1>\cr...\cr<case n>&<condition n>}`
+  creates a list of variants (preceded by the brace $\{$) in the math mode.
+
+* \i matrix `\matrix{<a>&<b>...&<e>\cr...\cr<u>&<v>...&<z>}`
+  creates a matrix of given values in math mode (without brackets around it).
+  `\pmatrix{<data>}` does the same but with ().
+
+* \i displaylines `$$\displaylines{<formula 1>\cr...\cr<formula n>}$$`
+  prints more (centered) formulae in the display mode.
+
+* \i eqalign 
+  `$$\eqalign{<form.1 left>&<form.1 right>\cr...\cr<form.n left>&<form.n right>}$$`\hfil\break
+  prints more formulae aligned by `&` character in the display mode.
+
+* \x`\eqalignno` behaves like `\eqalign` but second `&` followed by a `<mark>` 
+  can be in some lines. These lines have `<mark>` in the right margin.
+  \x`\leqalignno` does the same as `\eqalignno` but `<mark>` is put to the
+  left margin.  
+}
+\enditems
+
+\raggedbottom
+
+\nonum\sec Index
+
+\iis LaTeX/macros {\LaTeX/ macros}
+\iis plain~TeX/macros {plain \TeX/ macros}
+\iis OpTeX {\OpTeX}
+\iis pdfTeX {pdf\TeX}
+\iis luaTeX {lua\TeX}
+\iis XeTeX {\XeTeX}
+\iis TeX/engines {\TeX/ engines}
+\iis -percent {{\code{\\\%}}}
+\iis -at {{\code{\\\&}}}
+\iis -dollar {{\code{\\\$}}}
+\iis -hash {{\code{\\\#}}}
+\iis -space {{\code{\\}{\tt\char9251}}}
+\iis -italiccorr {{\code{\\/}}}
+\iis -hyphen {{\code{\\-}}}
+\iis TeXlive {\TeX live}
+\iis baselineskiprule {\code{\\baselineskip} rule}
+\iis -comma {{\code{\\,}}}
+\iis -greater {{\code{\\>}}}
+\iis -column {{\code{\\;}}}
+\iis -exclam {{\code{\\!}}}
+
+{\let\Blue=\relax \typosize[9/11] \begmulti 3 \makeindex \endmulti}
+
+\normalbottom
+
+\vfill
+
+\noindent
+Petr Olšák {petr@olsak.net}\nl
+Czech Technical University in Prague\nl
+Version of the text: 0.2 (\the\year/\the\month/\the\day)
+
+\break
+
+\end