xenocara/doc/xorg-docs/general/input/XKB-Enhancing.xml

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<article>

  <articleinfo>

    <title>How to further enhance XKB configuration</title>
    <authorgroup>
      <author>
	<firstname>Kamil</firstname><surname>Toman</surname>
      </author>
      <author>
	<firstname>Ivan</firstname><othername>U.</othername>
	<surname>Pascal</surname>
      </author>
    </authorgroup>
    <pubdate>25 November 2002</pubdate>
    <abstract>

      <para>
This guide is aimed to relieve one's labour to create a new (internationalized)
keyboard layout. Unlike other documents this guide accents the keymap developer's point of view.
      </para>

    </abstract>

  </articleinfo>

  <sect1>
    <title>Overview</title>

    <para>
The developer of a new layout should read the xkb
protocol specification (<ulink
url="http://www.x.org/docs/XKB/XKBproto.pdf"
>The X Keyboard Extension: Protocol Specification</ulink
>) at least
to clarify for himself some xkb-specific terms used in this document
and elsewhere in xkb configuration. Also it shows wise to understand
how the X server and a client digest their keyboard inputs
(with and without xkb).
    </para>

    <para>
A useful source is also <ulink
url="http://www.tsu.ru/~pascal/en/xkb"
>Ivan  Pascal's text about xkb configuration</ulink
> often referenced throughout this
document.
    </para>

    <para>
Note that this document covers only enhancements which
are to be made to XFree86 version 4.3 and X11R6.7.0 and above.
    </para>

  </sect1>

  <sect1>
    <title>The Basics</title>

    <para>
At the startup (or at later at user's command) X server starts its xkb
keyboard module extension and reads data from a compiled configuration
file.
    </para>

    <para>
This compiled configuration file is prepared by the
program <command>xkbcomp</command> which behaves altogether as an
ordinary compiler (see <userinput>man xkbcomp</userinput>).
Its input are human readable xkb configuration files which are verified and
then composed into a useful xkb configuration. Users don't need to mess with
<command>xkbcomp</command> themselves, for them it is invisible. Usually,
it is started upon X server startup.
    </para>

    <para>
As you probably already know, the xkb configuration consists of five
main modules:
      <variablelist>

	<varlistentry>
	  <term>Keycodes</term>
	  <listitem>
	    <para>
Tables that defines translation from keyboard scan codes into reasonable
symbolic names, maximum, minimum legal keycodes, symbolic aliases and
description of physically present LED-indicators. The primary sence of
this component is to allow definitions of maps of symbols (see below)
to be independent of physical keyboard scancodes. There are two main
naming conventions for symbolic names (always four bytes long):

	      <itemizedlist>
		<listitem>
		  <para>
 names which express some traditional meaning like
<keycode>&lt;SPCE&gt;</keycode> (stands for space bar) or
		  </para>
		</listitem>

		<listitem>
		  <para>
 names which express some relative positioning on a keyboard, for
example <keycode>&lt;AE01&gt;</keycode> (an exclamation mark on US keyboards), on
the right there are keys <keycode>&lt;AE02&gt;</keycode>, <keycode>&lt;AE03&gt;</keycode>
etc.
		  </para>
		</listitem>
	      </itemizedlist>

	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Types</term>
	  <listitem>
	    <para>
Types describe how the produced key is changed by active modifiers (like
Shift, Control, Alt, ...). There are several predefined types which
cover most of used combinations.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Compat</term>
	  <listitem>
	    <para>
Compatibility component defines internal behaviour of modifiers. Using
compat component you can assign various actions (elaborately described
in xkb specification) to key events. This is also the place where
LED-indicators behaviour is defined.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Symbols</term>
	  <listitem>
	    <para>
For i18n purposes, this is the most important table. It defines what
values (=symbols) are assigned to what keycodes (represented by their
symbolic name, see above). There may be defined more than one value
for each key and then it depends on a key type and on modifiers state
(respective compat component) which value will be the resulting one.
	    </para>
	  </listitem>
	</varlistentry>
	<varlistentry>
	  <term>Geometry</term>
	  <listitem>
	    <para>
Geometry files aren't used by xkb itself but they may be used by some
external programs to depict a keyboard image.
	    </para>
	  </listitem>
	</varlistentry>
      </variablelist>
All these components have the files located in xkb configuration tree
in subdirectories with the same names (usually in
<filename class="directory">/usr/lib/X11/xkb</filename>).
    </para>

  </sect1>

  <sect1>
    <title>Enhancing XKB Configuration</title>

    <para>
Most of xkb enhancements concerns a need to define new output symbols
for the some input key events. In other words, a need to define a new
symbol map (for a new language, standard or just to feel more comfortable
when typing text).
    </para>

    <para>
What do you need to do? Generally, you have to define following things:

      <itemizedlist>
	<listitem>
	  <para>
 the map of symbols itself
	  </para>
	</listitem>
	<listitem>
	  <para>
 the rules to allow users to select the new mapping
	  </para>
	</listitem>
	<listitem>
	  <para>
 the description of the new layout
	  </para>
	</listitem>
      </itemizedlist>

    </para>

    <para>
First of all, it is good to go through existing layouts and to examine
them if there is something you could easily adjust to fit your needs.
Even if there is nothing similar you may get some ideas about basic
concepts and used tricks.
    </para>

    <sect2>
      <title>Levels And Groups</title>

      <para>
Since XFree86 4.3.0 and X11R6.7.0 you can use
<firstterm>multi-layout</firstterm> concept of xkb
configuration.
Though it is still in boundaries of xkb protocol and general ideas, the
keymap designer must obey new rules when creating new maps. In exchange
we get a more powerful and cleaner configuration system.
      </para>

      <para>
Remember that it is the application which must decide which symbol
matches which keycode according to effective modifier state. The X server
itself sends only an input event message to. Of course, usually
the general interpretation is processed by Xlib, Xaw, Motif, Qt, Gtk
and similar libraries. The X server only supplies its mapping table
(usually upon an application startup).
      </para>

      <para>
You can think of the X server's symbol table as of a irregular table where each
keycode has its row and where each combination of modifiers determines exactly
one column. The resulting cell then gives the proper symbolic value. Not all
keycodes need to bind different values for different combination of modifiers.
<keycode>&lt;ENTER&gt;</keycode> key, for instance, usually doesn't depend on any
modifiers so it its row has only one column defined.
      </para>

      <para>
Note that in XKB there is no prior assumption that certain modifiers are bound
to certain columns. By editing proper files (see <xref linkend="keytypes" />)
this mapping can be changed as well.
      </para>

      <para>
Unlike the original X protocol the XKB approach is far more
flexible. It is comfortable to add one additional XKB term - group. You can
think of a group as of a vector of columns per each keycode (naturally the
dimension of this vector may differ for different keycodes). What is it good
for? The group is not very useful unless you intend to use more than one
logically different set of symbols (like more than one alphabet) defined in a
single mapping table. But then, the group has a natural meaning - each symbol
set has its own group and changing it means selecting a different one.
XKB approach allows up to four different groups. The columns inside each group
are called (shift) levels. The X server knows the current group and reports it
together with modifier set and with a keycode in key events.
      </para>

      <para>
To sum it up:

	<itemizedlist>
	  <listitem>
	    <para>
 for each keycode XKB keyboard map contains up to four one-dimensional
tables - groups (logically different symbol sets)
	    </para>
	  </listitem>
	  <listitem>
	    <para>
 for each group of a keycode XKB keyboard map contains some columns
- shift levels (values reached by combinations of Shift, Ctrl, Alt, ...
modifiers)
	    </para>
	  </listitem>
	  <listitem>
	    <para>
 different keycodes can have different number of groups
	    </para>
	  </listitem>
	  <listitem>
	    <para>
 different groups of one keycode can have different number of shift levels
	    </para>
	  </listitem>
	  <listitem>
	    <para>
 the current group number is tracked by X server
	    </para>
	  </listitem>
	</itemizedlist>
      </para>

      <para>
It is clear that if you sanely define levels, groups and sanely bind
modifiers and associated actions you can have simultaneously loaded up to
four different symbol sets where each of them would reside in its own group.
      </para>

      <para>
The multi-layout concept provides a facility to manipulate xkb groups
and symbol definitions in a way that allows almost arbitrary composition of
predefined symbol tables. To keep it fully functional you have to:

	<itemizedlist>
	  <listitem>
	    <para>
 define all symbols only in the first group
	    </para>
	  </listitem>
	  <listitem>
	    <para>
 (re)define any modifiers with extra care to avoid strange (anisometric)
behaviour
	    </para>
	  </listitem>
	</itemizedlist>

      </para>

    </sect2>

  </sect1>

  <sect1>
    <title>Defining New Layouts</title>

<!--
  TODO: It may be better to merge IP01 docs and this guide.
-->

    <para>
See <ulink url="http://www.tsu.ru/~pascal/en/xkb/internals.html"
>Some Words  About XKB internals</ulink
> for explanation of used xkb terms and problems
addressed by XKB extension.
    </para>

    <para>
See <ulink url="http://www.tsu.ru/~pascal/en/xkb/gram-common.html"
>Common  notes about XKB configuration files language</ulink
> for more precise explanation of
syntax of xkb configuration files.
    </para>

    <sect2>
      <title>Predefined XKB Symbol Sets</title>

      <para>
If you are about to define some European symbol map extension, you might
want to use on of four predefined latin alphabet layouts.
      </para>

<!--
    TODO: more details
    TODO: something similiar for phonetic layouts
    TODO: what are pc/pc layouts good for???
-->

      <para>
Okay, let's assume you want extend an existing keymap and you want to override
a few keys. Let's take a simple U.K. keyboard as an example (defined in
<filename>pc/gb</filename>):

	<screen>
partial default alphanumeric_keys
xkb_symbols "basic" {
  include "pc/latin"

  name[Group1]="Great Britain";

  key &#60;AE02&#62;  { [         2,   quotedbl,  twosuperior,    oneeighth ] };
  key &#60;AE03&#62;  { [         3,   sterling, threesuperior,    sterling ] };
  key &#60;AC11&#62;  { [apostrophe,         at, dead_circumflex, dead_caron] };
  key &#60;TLDE&#62;  { [     grave,    notsign,          bar,          bar ] };
  key &#60;BKSL&#62;  { [numbersign, asciitilde,   dead_grave,   dead_breve ] };
  key &#60;RALT&#62;  { type[Group1]="TWO_LEVEL",
                [ ISO_Level3_Shift, Multi_key ]   };

  modifier_map Mod5   { &#60;RALT&#62; };
};
	</screen>

      </para>

<!--
    TODO: ref IP01 file syntax TODO: some words about symbolic names like
    'sterling' and also about
     TODO: unicode characters (for non-latin alphabets),
     TODO: ref to compatibility symbolic names vs. unicode
-->

      <para>
It defines a new layout in <literal remap="tt">basic</literal> variant as an extension of common
latin alphabet layout. The layout (symbol set) name is set to "Great Britain".
Then there are redefinitions of a few keycodes and a modifiers binding. As you
can see the number of shift levels is the same for
<keycode>&lt;AE02&gt;</keycode>, <keycode>&lt;AE03&gt;</keycode>,
<keycode>&lt;AC11&gt;</keycode>, <keycode>&lt;TLDE&gt;</keycode> and
<keycode>&lt;BKSL&gt;</keycode> keys but it differs from number of shift
levels of <keycode>&lt;RALT&gt;</keycode>.
      </para>

      <para>
Note that the <keycode>&lt;RALT&gt;</keycode> key itself is a binding key for Mod5 and
that it
serves like a shift modifier for LevelThree, together with Shift
as a multi-key. It is a good habit to respect this rule in a new similar
layout.
      </para>

      <para>
Okay, you could now define more variants of your new layout besides
<literal remap="tt">basic</literal> simply by including (augmenting/overriding/...) the basic
definition and altering what may be needed.
      </para>

    </sect2>

    <sect2 id="keytypes" xreflabel="keytypes">
      <title>Key Types</title>

      <para>
The differences in the number of columns (shift levels) are caused by
a different types of keys (see the types definition in section basics).  Most
keycodes have implicitly set the keytype in the included
<quote><filename>pc/latin</filename></quote> file to
<quote><literal remap="tt">FOUR&lowbar;LEVEL&lowbar;ALPHABETIC</literal></quote>. The only exception is
<keycode>&lt;RALT&gt;</keycode> keycode which is explicitly set
<quote><literal remap="tt">TWO&lowbar;LEVEL</literal></quote> keytype.
      </para>

      <para>
All those names refer to pre-defined shift level schemes. Usually you can
choose a suitable shift level scheme from <literal remap="tt">default</literal> types scheme list
in proper xkb component's subdirectory.
      </para>

      <para>
The most used schemes are:
	<variablelist>
	  <varlistentry>
	    <term>ONE&lowbar;LEVEL</term>
	    <listitem>
	      <para>
The key does not depend on any modifiers. The symbol from first level
is always chosen.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>TWO&lowbar;LEVEL</term>
	    <listitem>
	      <para>
The key uses a modifier Shift and may have two possible values.
The second level may be chosen by Shift modifier. If Lock modifier
(usually Caps-lock) applies the symbol is further processed using
system-specific capitalization rules. If both Shift+Lock modifier apply the
symbol from the second level is taken and capitalization rules are applied
(and usually have no effect).
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>ALPHABETIC</term>
	    <listitem>
	      <para>
The key uses modifiers Shift and Lock. It may have two possible
values. The second level may be chosen by Shift modifier. When Lock
modifier applies, the symbol from the first level is taken and further
processed using system-specific capitalization rules. If both Shift+Lock
modifier apply the symbol from the first level is taken and no
capitalization rules applied. This is often called shift-cancels-caps
behaviour.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>THREE&lowbar;LEVEL</term>
	    <listitem>
	      <para>
Is the same as TWO&lowbar;LEVEL but it considers an extra modifier -
LevelThree which can be used to gain the symbol value from the third
level. If both Shift+LevelThree modifiers apply the value from the third
level is also taken. As in TWO&lowbar;LEVEL, the Lock modifier doesn't influence
the resulting level. Only Shift and LevelThree are taken into that
consideration. If the Lock modifier is active capitalization rules
are applied on the resulting symbol.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>FOUR&lowbar;LEVEL</term>
	    <listitem>
	      <para>
Is the same as THREE&lowbar;LEVEL but unlike LEVEL&lowbar;THREE if both Shift+LevelThree
modifiers apply the symbol is taken from the fourth level.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>FOUR&lowbar;LEVEL&lowbar;ALPHABETIC</term>
	    <listitem>
	      <para>
Is similar to FOUR&lowbar;LEVEL but also defines shift-cancels-caps behaviour
as in ALPHABETIC. If Lock+LevelThree apply the symbol from the
third level is taken and the capitalization rules are applied.
If Lock+Shift+LevelThree apply the symbol from the third level is taken
and no capitalization rules are applied.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>KEYPAD</term>
	    <listitem>
	      <para>
As the name suggest this scheme is primarily used for numeric keypads.
The scheme considers two modifiers - Shift and NumLock. If none
of modifiers applies the symbol from the first level is taken. If either
Shift or NumLock modifiers apply the symbol from the second level is taken.
If both Shift+NumLock modifiers apply the symbol from the first level
is taken. Again, shift-cancels-caps variant.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>FOUR&lowbar;LEVEL&lowbar;KEYPAD</term>
	    <listitem>
	      <para>
Is similar to KEYPAD scheme but considers also LevelThree modifier.
If LevelThree modifier applies the symbol from the third level is taken.
If Shift+LevelThree or NumLock+LevelThree apply the symbol from the fourth
level is taken. If all Shift+NumLock+LevelThree modifiers apply the symbol
from the third level is taken. This also, shift-cancels-caps variant.
	      </para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </para>

      <para>
Besides that, there are several schemes for special purposes:
	<variablelist>

	  <varlistentry>
	    <term>PC&lowbar;BREAK</term>
	    <listitem>
	      <para>
It is similar to TWO&lowbar;LEVEL scheme but it considers the Control
modifier rather than Shift. That means, the symbol from the second level
is chosen by Control rather than by Shift.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>PC&lowbar;SYSRQ</term>
	    <listitem>
	      <para>
It is similar to TWO&lowbar;LEVEL scheme but it considers the Alt modifier rather
than Shift. That means, the symbol from the second level
is chosen by Alt rather than by Shift.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>CTRL+ALT</term>
	    <listitem>
	      <para>
The key uses modifiers Alt and Control. It may have two possible
values. If only one modifier (Alt or Control) applies the symbol
from the first level is chosen. Only if both Alt+Control modifiers apply
the symbol from the second level is chosen.
	      </para>
	    </listitem>
	  </varlistentry>
	  <varlistentry>
	    <term>SHIFT+ALT</term>
	    <listitem>
	      <para>
The key uses modifiers Shift and Alt. It may have two possible values.
If only one modifier (Alt or Shift) applies the symbol
from the first level is chosen. Only if both Alt+Shift modifiers apply
the symbol from the second level is chosen.
	      </para>
	    </listitem>
	  </varlistentry>
	</variablelist>
      </para>

      <para>
If needed, special <literal remap="tt">caps</literal> schemes may be used.
They redefine the standard behaviour of all
<literal remap="tt">*ALPHABETIC</literal> types. The layouts (maps of
symbols) with keys defined in respective types then automatically change
their behaviour accordingly. Possible redefinitions are:

	<itemizedlist>
	  <listitem><para>internal</para></listitem>
	  <listitem><para>internal&lowbar;nocancel</para></listitem>
	  <listitem><para>shift</para></listitem>
	  <listitem><para>shift&lowbar;nocancel</para></listitem>
	</itemizedlist>

None of these schemes should be used directly. They are defined merely
for <literal remap="tt">'caps:'</literal> xkb options (used to globally
change the layouts behaviour).
      </para>

      <para>
Don't alter any of existing key types. If you need a different behaviour
create a new one.
      </para>

      <sect3>
	<title>More On Definitions Of Types</title>

	<para>
When the XKB software deals with a separate type description it gets
a complete list of modifiers that should be taken into account from the
<literal remap="tt">'modifiers=&lt;list of modifiers&gt;'</literal> list and expects that a set
of <literal remap="tt">'map[&lt;combination of modifiers&gt;]=&lt;list of modifiers&gt;'</literal>
instructions that contain the mapping for each combination of modifiers
mentioned in that list. Modifiers that are not explicitly listed are NOT taken
into account
when the resulting shift level is computed.
If some combination is omitted the program (subroutine) should choose the first
level for this combination (a quite reasonable behavior).
	</para>

	<para>
Lets consider an example with two modifiers <keysym>ModOne</keysym> and
<keysym>ModTwo</keysym>:

	  <screen>
type "..." {
    modifiers = ModOne+ModTwo;
    map[None] = Level1;
    map[ModOne] = Level2;
};
	  </screen>

In this case the map statements for <keysym>ModTwo</keysym> only and
<keysym>ModOne+ModTwo</keysym> are omitted.  It means that if
the <keysym>ModTwo</keysym> is active the subroutine can't found
explicit mapping for such combination an will use
the <emphasis>default level</emphasis> i.e. Level1.
	</para>

	<para>
But in the case the type described as:

	  <screen>
type "..." {
    modifiers = ModOne;
    map[None] = Level1;
    map[ModOne] = Level2;
};
	  </screen>

the ModTwo will not be taken into account and the resulting level depends on
the ModOne state only. That means, ModTwo alone produces the Level1 but the
combination ModOne+ModTwo produces the Level2 as well as ModOne alone.
	</para>

	<para>
What does it mean if the second modifier is the Lock? It means that in
the first case (the Lock itself is included in the list of modifiers but
combinations with this modifier aren't mentioned in the map statements)
the internal capitalization rules will be applied to the symbol from the first
level. But in the second case the capitalization will be applied to the symbol
chosen accordingly to he first modifier - and this can be the symbol from the
first as well as from the second level.
	</para>

	<para>
Usually, all modifiers introduced in <literal remap="tt">'modifiers=&lt;list of modifiers&gt;'</literal> list are used for shift level calculation and then
discarded. Sometimes this is not desirable. If you want to use a modifier
for shift level calculation but you don't want to discard it, you may
list in '<literal remap="tt">preserve[&lt;combination of modifiers&gt;]=&lt;list of modifiers&gt;'</literal>. That means, for a given combination all listed modifiers
will be preserved. If the Lock modifier is preserved then the resulting
symbol is passed to internal capitalization routine regardless whether
it has been used for a shift level calculation or not.
	</para>

	<para>
Any key type description can use both real and virtual modifiers. Since real
modifiers always have standard names it is not necessary to explicitly declare
them. Virtual modifiers can have arbitrary names and can be declared (prior
using them) directly in key type definition:

	  <screen>
virtual_modifiers &lt;comma-separated list of modifiers&gt;  ;
	  </screen>

as seen in for example <literal remap="tt">basic</literal>, <literal remap="tt">pc</literal> or <literal remap="tt">mousekeys</literal> key
type definitions.
	</para>

      </sect3>

    </sect2>

    <sect2>
      <title>Rules</title>

      <para>
Once you are finished with your symbol map you need to add it
to rules file. The rules file describes how all the
five basic keycodes, types, compat, symbols and geometry components
should be composed to give a sensible resulting xkb configuration.
      </para>

      <para>
The main advantage of rules over formerly used keymaps is a possibility
to simply parameterize (once) fixed patterns of configurations and thus
to elegantly allow substitutions of various local configurations
into predefined templates.
      </para>

      <para>
A pattern in a rules file (often located in
<filename>/usr/lib/X11/xkb/rules</filename>)
 can be parameterized with four other arguments:
<literal remap="tt">Model</literal>, <literal remap="tt">Layout</literal>,
<literal remap="tt">Variant</literal> and <literal remap="tt">Options</literal>.
For most cases parameters <literal remap="tt">model</literal> and
<literal remap="tt">layout</literal> should
be sufficient for choosing a functional keyboard mapping.
      </para>

      <para>
The rules file itself is composed of pattern lines and lines with rules. The
pattern line starts with an exclamation mark ('<literal remap="tt">!</literal>')
and describes how will the xkb interpret the following lines (rules). A sample
rules file looks like this:

	<screen>
! model 	                =	keycodes
  macintosh_old	                =	macintosh
  ...
  *		                =	xorg

! model		                =	symbols
  hp		                =	+inet(&percnt;m)
  microsoftpro	                =	+inet(&percnt;m)
  geniuscomfy	                =	+inet(&percnt;m)

! model	    layout[1]	        =	symbols
  macintosh	us	        =	macintosh/us&percnt;(v[1])
  *             *               =       pc/pc(&percnt;m)+pc/&percnt;l[1]&percnt;(v[1])

! model     layout[2]	        =	symbols
  macintosh     us              =	+macintosh/us[2]&percnt;(v[2]):2
  *		*               =	+pc/&percnt;l[2]&percnt;(v[2]):2

! option			=	types
  caps:internal			=	+caps(internal)
  caps:internal_nocancel	=	+caps(internal_nocancel)
	</screen>

      </para>

      <para>
Each rule defines what certain combination of values on the left side
of equal sign ('<literal remap="tt">=</literal>') results in. For
example a (keyboard) model <literal remap="tt">macintosh&lowbar;old</literal>
instructs xkb to take definitions of keycodes from
file <filename>keycodes/macintosh</filename> while the rest
of models (represented by a wild card '<literal remap="tt">*</literal>')
instructs it to take them from file <filename>keycodes/xorg</filename>.
The wild card represents all possible values on the left side which
were not found in any of the previous rules.  The more specialized
(more complete) rules have higher precedence than general ones,
i.e. the more general rules supply reasonable default values.
      </para>

      <para>
As you can see some lines contain substitution parameters - the parameters
preceded by the percent sign ('<literal remap="tt">&percnt;</literal>').
The first alphabetical character after the percent sign expands to the
value which has been found on the left side. For
example <literal remap="tt">+&percnt;l&percnt;(v)</literal> expands
into <literal remap="tt">+cz(bksl)</literal> if the respective values
on the left side were <literal remap="tt">cz</literal> layout in
its <literal remap="tt">bksl</literal> variant. More, if the layout
resp. variant parameter is followed by a pair of brackets
('<literal remap="tt">[</literal>', '<literal remap="tt">]</literal>')
it means that xkb should <emphasis>place the layout resp. variant into
specified xkb group</emphasis>. If the brackets are omitted the first
group is the default value.
      </para>

      <para>
So the second block of rules enhances symbol definitions for some particular
keyboard models with extra keys (for internet, multimedia, ...) . Other models
are left intact. Similarly, the last block overrides some key type definitions,
so the common global behaviour ''shift cancels caps'' or ''shift doesn't cancel
caps'' can be selected. The rest of rules produces special symbols for each
variant <literal remap="tt">us</literal> layout of
<literal remap="tt">macintosh</literal> keyboard and standard pc
symbols in appropriate variants as a default.
      </para>

    </sect2>

<!--
  TODO: more words about group switching (XkbOptions grp:...)?
-->

<!--
  TODO: user & 3rd party xkb tree?
  TODO: better and more complex explanation of rules
-->

    <sect2>
      <title>Descriptive Files of Rules</title>

      <para>
Now you just need to add a detailed description to
<filename>&lt;rules&gt;.xml</filename>
description file so the other users (and external programs which often parse
this file) know what is your work about.
      </para>

<!--
    TODO: format and semantics
-->

      <sect3>
	<title>Old Descriptive Files</title>

	<para>
The formerly used descriptive files were named <filename>&lt;rules&gt;.lst</filename>
Its structure is very simple and quite self descriptive but such simplicity
had also some cavities, for example there was no way how to describe local
variants of layouts and there were problems with the localization of
descriptions. To preserve compatibility with some older programs,
new XML descriptive files can be converted to old format '.lst'.
	</para>

	<para>
For each parameter of rules file should be described its meaning. For the rules
file described above the <filename>.lst</filename> file could look like:

	  <screen>
! model
  pc104		Generic 104-key PC
  microsoft	Microsoft Natural
  pc98		PC-98xx Series
  macintosh     Original Macintosh
  ...

! layout
  us		U.S. English
  cz		Czech
  de		German
  ...

! option
  caps:internal		 uses internal capitalization. Shift cancels Caps
  caps:internal_nocancel uses internal capitalization. Shift doesn't cancel Caps

	  </screen>

	</para>

	<para>
And that should be it. Enjoy creating your own xkb mapping.
	</para>

      </sect3>

    </sect2>

  </sect1>

</article>