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+                  How to further enhance XKB configuration
+
+                         Kamil Toman, Ivan U. Pascal
+
+                              25 November 2002
+
+                                  Abstract
+
+     This guide is aimed to relieve one's labour to create a new (inter-
+     nationalized) keyboard layout. Unlike other documents this guide
+     accents the keymap developer's point of view.
+
+1.  Overview
+
+The developer of a new layout should read the xkb protocol specification (The
+X Keyboard Extension: Protocol Specification <URL:http://www.x-
+docs.org/XKB/XKBproto.pdf>) 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).
+
+A useful source is also Ivan  Pascal's text about xkb configuration
+<URL:http://www.tsu.ru/~pascal/en/xkb> often referenced throughout this docu-
+ment.
+
+Note that this document covers only enhancements which are to be made to
+XFree86 version 4.3.x and above.
+
+2.  The Basics
+
+At the startup (or at later at user's command) X server starts its xkb key-
+board module extension and reads data from a compiled configuration file.
+
+This compiled configuration file is prepared by the program xkbcomp which
+behaves altogether as an ordinary compiler (see man xkbcomp).  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 xkbcomp them-
+selves, for them it is invisible. Usually, it is started upon X server
+startup.
+
+As you probably already know, the xkb configuration consists of five main
+modules:
+
+      Keycodes
+            Tables that defines translation from keyboard scan codes into
+            reasonable symbolic names, maximum, minimum legal keycodes, sym-
+            bolic aliases and description of physically present LED-indica-
+            tors. The primary sence of this component is to allow definitions
+            of maps of symbols (see below) to be independent of physical key-
+            board scancodes. There are two main naming conventions for sym-
+            bolic names (always four bytes long):
+
+               o  names which express some traditional meaning like <SPCE>
+                 (stands for space bar) or
+
+               o  names which express some relative positioning on a key-
+                 board, for example <AE01> (an exclamation mark on US key-
+                 boards), on the right there are keys <AE02>, <AE03> etc.
+
+      Types
+            Types describe how the produced key is changed by active modi-
+            fiers (like Shift, Control, Alt, ...). There are several prede-
+            fined types which cover most of used combinations.
+
+      Compat
+            Compatibility component defines internal behaviour of modifiers.
+            Using compat component you can assign various actions (elabo-
+            rately described in xkb specification) to key events. This is
+            also the place where LED-indicators behaviour is defined.
+
+      Symbols
+            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.
+
+      Geometry
+            Geometry files aren't used by xkb itself but they may be used by
+            some external programs to depict a keyboard image.
+
+All these components have the files located in xkb configuration tree in sub-
+directories with the same names (usually in /usr/lib/X11/xkb).
+
+3.  Enhancing XKB Configuration
+
+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).
+
+What do you need to do? Generally, you have to define following things:
+
+   o  the map of symbols itself
+
+   o  the rules to allow users to select the new mapping
+
+   o  the description of the new layout
+
+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.
+
+3.1  Levels And Groups
+
+Since XFree86 4.3.0 you can use multi-layout 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.
+
+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 inter-
+pretation is processed by Xlib, Xaw, Motif, Qt, Gtk and similar libraries.
+The X server only supplies its mapping table (usually upon an application
+startup).
+
+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.  <ENTER> key, for instance, usually doesn't depend on any modi-
+fiers so it its row has only one column defined.
+
+Note that in XKB there is no prior assumption that certain modifiers are
+bound to certain columns. By editing proper files (see keytypes (section 4.2,
+page 1)) this mapping can be changed as well.
+
+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 ta-
+ble. 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.
+
+To sum it up:
+
+   o  for each keycode XKB keyboard map contains up to four one-dimensional
+     tables - groups (logically different symbol sets)
+
+   o  for each group of a keycode XKB keyboard map contains some columns -
+     shift levels (values reached by combinations of Shift, Ctrl, Alt, ...
+     modifiers)
+
+   o  different keycodes can have different number of groups
+
+   o  different groups of one keycode can have different number of shift lev-
+     els
+
+   o  the current group number is tracked by X server
+
+It is clear that if you sanely define levels, groups and sanely bind modi-
+fiers and associated actions you can have simultaneously loaded up to four
+different symbol sets where each of them would reside in its own group.
+
+The multi-layout concept provides a facility to manipulate xkb groups and
+symbol definitions in a way that allows almost arbitrary composition of pre-
+defined symbol tables. To keep it fully functional you have to:
+
+   o  define all symbols only in the first group
+
+   o  (re)define any modifiers with extra care to avoid strange (anisometric)
+     behaviour
+
+4.  Defining New Layouts
+
+See Some Words  About XKB internals <URL:http://www.tsu.ru/~pas-
+cal/en/xkb/internals.html> for explanation of used xkb terms and problems
+addressed by XKB extension.
+
+See Common  notes about XKB configuration files language
+<URL:http://www.tsu.ru/~pascal/en/xkb/gram-common.html> for more precise
+explanation of syntax of xkb configuration files.
+
+4.1  Predefined XKB Symbol Sets
+
+If you are about to define some European symbol map extension, you might want
+to use on of four predefined latin alphabet layouts.
+
+Okay, let's assume you want extend an existing keymap and you want to over-
+ride a few keys. Let's take a simple U.K. keyboard as an example (defined in
+pc/gb):
+
+     partial default alphanumeric_keys
+     xkb_symbols "basic" {
+       include "pc/latin"
+
+       name[Group1]="Great Britain";
+
+       key <AE02>  { [         2,   quotedbl,  twosuperior,    oneeighth ] };
+       key <AE03>  { [         3,   sterling, threesuperior,    sterling ] };
+       key <AC11>  { [apostrophe,         at, dead_circumflex, dead_caron] };
+       key <TLDE>  { [     grave,    notsign,          bar,          bar ] };
+       key <BKSL>  { [numbersign, asciitilde,   dead_grave,   dead_breve ] };
+       key <RALT>  { type[Group1]="TWO_LEVEL",
+                     [ ISO_Level3_Shift, Multi_key ]   };
+
+       modifier_map Mod5   { <RALT> };
+     };
+
+It defines a new layout in basic 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 <AE02>, <AE03>,
+<AC11>, <TLDE> and <BKSL> keys but it differs from number of shift levels of
+<RALT>.
+
+Note that the <RALT> 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.
+
+Okay, you could now define more variants of your new layout besides basic
+simply by including (augmenting/overriding/...) the basic definition and
+altering what may be needed.
+
+4.2  Key Types
+
+The differences in the number of columns (shift levels) are caused by a dif-
+ferent types of keys (see the types definition in section basics).  Most key-
+codes have implicitly set the keytype in the included "pc/latin" file to
+"FOUR_LEVEL_ALPHABETIC". The only exception is <RALT> keycode which is
+explicitly set "TWO_LEVEL" keytype.
+
+All those names refer to pre-defined shift level schemes. Usually you can
+choose a suitable shift level scheme from default types scheme list in proper
+xkb component's subdirectory.
+
+The most used schemes are:
+
+      ONE_LEVEL
+            The key does not depend on any modifiers. The symbol from first
+            level is always chosen.
+
+      TWO_LEVEL
+            The key uses a modifier Shift and may have two possible values.
+            The second level may be chosen by Shift modifier. If Lock modi-
+            fier (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 cap-
+            italization rules are applied (and usually have no effect).
+
+      ALPHABETIC
+            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.
+
+      THREE_LEVEL
+            Is the same as TWO_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_LEVEL, the Lock
+            modifier doesn't influence the resulting level. Only Shift and
+            LevelThree are taken into that consideration. If the Lock modi-
+            fier is active capitalization rules are applied on the resulting
+            symbol.
+
+      FOUR_LEVEL
+            Is the same as THREE_LEVEL but unlike LEVEL_THREE if both
+            Shift+LevelThree modifiers apply the symbol is taken from the
+            fourth level.
+
+      FOUR_LEVEL_ALPHABETIC
+            Is similar to FOUR_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.
+
+      KEYPAD
+            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.
+
+      FOUR_LEVEL_KEYPAD
+            Is similar to KEYPAD scheme but considers also LevelThree modi-
+            fier.  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+Num-
+            Lock+LevelThree modifiers apply the symbol from the third level
+            is taken. This also, shift-cancels-caps variant.
+
+Besides that, there are several schemes for special purposes:
+
+      PC_BREAK
+            It is similar to TWO_LEVEL scheme but it considers the Control
+            modifier rather than Shift. That means, the symbol from the sec-
+            ond level is chosen by Control rather than by Shift.
+
+      PC_SYSRQ
+            It is similar to TWO_LEVEL scheme but it considers the Alt modi-
+            fier rather than Shift. That means, the symbol from the second
+            level is chosen by Alt rather than by Shift.
+
+      CTRL+ALT
+            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 modi-
+            fiers apply the symbol from the second level is chosen.
+
+      SHIFT+ALT
+            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.
+
+If needed, special caps schemes may be used. They redefine the standard
+behaviour of all *ALPHABETIC types. The layouts (maps of symbols) with keys
+defined in respective types then automatically change their behaviour accord-
+ingly. Possible redefinitions are:
+
+   o internal
+
+   o internal_nocancel
+
+   o shift
+
+   o shift_nocancel
+
+None of these schemes should be used directly. They are defined merely for
+'caps:' xkb options (used to globally change the layouts behaviour).
+
+Don't alter any of existing key types. If you need a different behaviour cre-
+ate a new one.
+
+4.2.1  More On Definitions Of Types
+
+When the XKB software deals with a separate type description it gets a com-
+plete list of modifiers that should be taken into account from the 'modi-
+fiers=<list of modifiers>' list and expects that a set of 'map[<combination
+of modifiers>]=<list of modifiers>' 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).
+
+Lets consider an example with two modifiers ModOne and ModTwo:
+
+     type "..." {
+         modifiers = ModOne+ModTwo;
+         map[None] = Level1;
+         map[ModOne] = Level2;
+     };
+
+In this case the map statements for ModTwo only and ModOne+ModTwo are omit-
+ted.  It means that if the ModTwo is active the subroutine can't found
+explicit mapping for such combination an will use the default level i.e.
+Level1.
+
+But in the case the type described as:
+
+     type "..." {
+         modifiers = ModOne;
+         map[None] = Level1;
+         map[ModOne] = Level2;
+     };
+
+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.
+
+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 combina-
+tions 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.
+
+Usually, all modifiers introduced in 'modifiers=<list of modifiers>' 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 'preserve[<combination of modi-
+fiers>]=<list of modifiers>'. 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.
+
+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:
+
+     virtual_modifiers <comma-separated list of modifiers>  ;
+
+as seen in for example basic, pc or mousekeys key type definitions.
+
+4.3  Rules
+
+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, sym-
+bols and geometry components should be composed to give a sensible resulting
+xkb configuration.
+
+The main advantage of rules over formerly used keymaps is a possibility to
+simply parameterize (once) fixed patterns of configurations and thus to ele-
+gantly allow substitutions of various local configurations into predefined
+templates.
+
+A pattern in a rules file (often located in /usr/lib/X11/xkb/rules) can be
+parameterized with four other arguments: Model, Layout, Variant and Options.
+For most cases parameters model and layout should be sufficient for choosing
+a functional keyboard mapping.
+
+The rules file itself is composed of pattern lines and lines with rules. The
+pattern line starts with an exclamation mark ('!') and describes how will the
+xkb interpret the following lines (rules). A sample rules file looks like
+this:
+
+     ! model                   =   keycodes
+       macintosh_old                     =   macintosh
+       ...
+       *                       =   xfree86
+
+     ! model                        =   symbols
+       hp                      =   +inet(%m)
+       microsoftpro                 =   +inet(%m)
+       geniuscomfy                  =   +inet(%m)
+
+     ! model       layout[1]          = symbols
+       macintosh    us           = macintosh/us%(v[1])
+       *             *               =       pc/pc(%m)+pc/%l[1]%(v[1])
+
+     ! model     layout[2]            = symbols
+       macintosh     us              =  +macintosh/us[2]%(v[2]):2
+       *       *               =   +pc/%l[2]%(v[2]):2
+
+     ! option            =    types
+       caps:internal               =    +caps(internal)
+       caps:internal_nocancel =    +caps(internal_nocancel)
+
+Each rule defines what certain combination of values on the left side of
+equal sign ('=') results in. For example a (keyboard) model macintosh_old
+instructs xkb to take definitions of keycodes from file keycodes/macintosh
+while the rest of models (represented by a wild card '*') instructs it to
+take them from file keycodes/xfree86. 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 gen-
+eral ones, i.e. the more general rules supply reasonable default values.
+
+As you can see some lines contain substitution parameters - the parameters
+preceded by the percent sign ('%'). The first alphabetical character after
+the percent sign expands to the value which has been found on the left side.
+For example +%l%(v) expands into +cz(bksl) if the respective values on the
+left side were cz layout in its bksl variant. More, if the layout resp. vari-
+ant  parameter is followed by a pair of brackets ('[', ']') it means that xkb
+should place the layout resp. variant into specified xkb group. If the brack-
+ets are omitted the first group is the default value.
+
+So the second block of rules enhances symbol definitions for some particular
+keyboard models with extra keys (for internet, multimedia, ...) . Other mod-
+els are left intact. Similarly, the last block overrides some key type defi-
+nitions, 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 us layout of macintosh keyboard and standard pc sym-
+bols in appropriate variants as a default.
+
+4.4  Descriptive Files of Rules
+
+Now you just need to add a detailed description to <rules>.xml description
+file so the other users (and external programs which often parse this file)
+know what is your work about.
+
+4.4.1  Old Descriptive Files
+
+The formerly used descriptive files were named <rules>.lst Its structure is
+very simple and quite self descriptive but such simplicity had also some cav-
+ities, 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 con-
+verted to old format '.lst'.
+
+For each parameter of rules file should be described its meaning. For the
+rules file described above the .lst file could look like:
+
+     ! 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
+
+And that should be it. Enjoy creating your own xkb mapping.
+
+     Generated from XFree86: xc/programs/Xserver/hw/xfree86/doc/sgml/XKB-Enhancing.sgml,v 1.2 dawes Exp $
+
+
+$XFree86: xc/programs/xkbcomp/README.enhancing,v 1.5 2003/10/19 19:57:44 dawes Exp $