The xkb_keycodes section
========================

This is the simplest section type, and is the first one to be
compiled. The purpose of this is mostly to map between the
hardware/evdev scancodes and xkb keycodes. Each key is given a name
by which it can be referred to later, e.g. in the symbols section.

Keycode statements
------------------
Statements of the form:

    <TLDE> = 49;
    <AE01> = 10;

The above would let 49 and 10 be valid keycodes in the keymap, and
assign them the names TLDE and AE01 respectively. The format <WXYZ> is
always used to refer to a key by name.

[The naming convention <AE01> just denoted the position of the key
in the main alphanumric section of the keyboard, with the two letters
specifying the row and the two digits specifying the column, from
the bottom left.]

In the common case this just maps to the evdev scancodes from
/usr/include/linux/input.h, e.g. the following definitions:

     #define KEY_GRAVE            41
     #define KEY_1                2

correspond to the ones above. Similar definitions appear in the
xf86-input-keyboard driver. Note that in all current keymaps there's a
constant offset of 8 (for historical reasons).

If there's a conflict, like the same name given to different keycodes,
or same keycode given different names, it is resolved according to the
merge mode which applies to the definitions.

Alias statements
----------------
Statements of the form:

    alias <MENU> = <COMP>;

Allows to refer to a previously defined key (here <COMP>) by another
name (here <MENU>). Conflicts are handled similarly to keycode
statements.

LED name statements
-------------------
Statements of the form:

    indicator 1 = "Caps Lock";
    indicator 2 = "Num Lock";
    indicator 3 = "Scroll Lock";

Assigns a name to the keyboard LED (a.k.a indicator) with the given
index. The LED may be referred by this name later in the compat section
and by the user.


The xkb_types section
=====================

This section is the second to be processesed, after xkb_keycodes.
However, it is completely independent and could have been the first
to be processed (it does not refer to specific keys as specified in
the xkb_keycodes section).

This section defines key types, which, given a key and a keyboard
state (i.e. modifier state and group), determine the shift level to
be used in translating the key to keysyms. These types are assigned
to each group in each key, in the xkb_symbols section.

Key types are called this way because, in a way, they really describe
the "type" of the key (or more correctly, a specific group of the
key). For example, an ordinary keymap will provide a type called
"KEYPAD", which consists of two levels, with the second level being
chosen according to the state of the Num Lock (or Shift) modifiers.
Another example is a type called "ONE_LEVEL", which is usually
assigned to keys such as Escape; these have just one level and are
not affected by the modifier state. Yet more common examples are
"TWO_LEVEL" (with Shift choosing the second level), "ALPHABETIC"
(where Caps Lock may also choose the second level), etc.

Type definitions
----------------
Statements of the form:

    type "FOUR_LEVEL" { ... }

The above would create a new type named "FOUR_LEVEL".
The body of the definition may include statements of the following
forms:

- level_name statements (mandatory for each level in the type):

        level_name[Level1] = "Base";

  Gives each level in this type a descriptive name. It isn't used
  for anything.
  Note: A level may be specified as Level[1-8] or just a number (can
  be more than 8).

- modifiers statement (mandatory, should be specified only once):

        modifiers = Shift+Lock+LevelThree;

  A mask of real and virtual modifiers. These are the only modifiers
  being considered when matching the modifier state against the type.
  The other modifiers, whether active or not, are masked out in the
  calculation.

- map entry statements (should have at least as many mappings as there
  are levels in the type):

        map[Shift+LevelThree] = Level4;

  If the active modifiers, masked with the type's modifiers (as stated
  above), match (i.e. equal) the modifiers inside the map[] statement,
  then the level in the right hand side is chosen. For example, in the
  above, if in the current keyboard state the Shift and LevelThree
  modifiers are active, while the Lock modifier is not, then the
  keysym(s) in the 4th level of the group will be returned to the
  user.

- preserve statements:

        map[Shift+Lock+LevelThree] = Level5;
        preserve[Shift+Lock+LevelThree] = Lock;

  When a map entry matches the active modifiers and the level it
  specified is chosen, then these modifiers are said to be "consumed";
  for example, in a simple US keymap where the "g" key is assigned an
  ordinary ALPHABETIC key type, if the Lock (Caps Lock) modifier is
  active and the key is pressed, then a "G" keysym is produced (as
  opposed to lower-case "g"). This is because the type definition has
  a map entry like the following:

        map[Lock] = Level2;

  And as such the Lock modifier is consumed. This information is
  relevant for applications which further process the modifiers,
  since by then the consumed modifiers have already "done their part"
  and should be masked out.

  However, sometimes even if a modifier is actually used to choose
  the shift level (as Lock above), it should *not* be reported as
  consumed, for various reasons. In this case, a preserve[] statement
  can be used to augment the map entry. The modifiers inside the square
  brackets should match one of the map[] statements in the type. The
  right hand side should consists of modifiers from the left hand
  side; these modifiers are then "preserved" and not reported as
  consumed.


The xkb_compat section
======================

This section is the third to be processed, after xkb_keycodes and
xkb_types.

Interpret statements
--------------------
Statements of the form:

    interpret Num_Lock+Any { ... }
    interpret Shift_Lock+AnyOf(Shift+Lock) { ... }

The xkb_symbols section (see below) allows the keymap author to perform,
among other things, the following things for each key:

- Bind an action, like SetMods or LockGroup, to the key. Actions, like
  symbols, are specified for each level of each group in the key
  separately.

- Add a virtual modifier to the key's virtual modifier mapping (vmodmap).

- Specify whether the key should repeat or not.

However, doing this for each key (or level) is tedious and inflexible.
Interpret's are a mechanism to apply these settings to a bunch of
keys/levels at once.

Each interpret specifies a condition by which it attaches to certain
levels. The condition consists of two parts:

- A keysym. If the level has a different (or more than one) keysym, the
  match fails. Leaving out the keysym is equivalent to using the NoSymbol
  keysym, which always matches successfully.

- A modifier predicate. The predicate consists of a matching operation
  and a mask of (real) modifiers. The modifiers are matched against the
  key's modifier map (modmap). The matching operation can be one of the
  following:

  * AnyOfOrNone - The modmap must either be empty or include at least
    one of the specified modifiers.
  * AnyOf - The modmap must include at least one of the specified
    modifiers.
  * NoneOf - The modmap must not include any of the specified modifiers.
  * AllOf - The modmap must include all of the specified modifiers (but
    may include others as well).
  * Exactly - The modmap must be exactly the same as the specified
    modifiers.

  Leaving out the predicate is equivalent to using AnyOfOrNone while
  specifying all modifiers. Leaving out just the matching condition
  is equivalent to using Exactly.

An interpret may also include "useModMapMods = level1;" - see below.

If a level fulfils the conditions of several interpret's, only the
most specific one is used:

- A specific keysym will always match before a generic NoSymbol
  condition.

- If the keysyms are the same, the interpret with the more specific
  matching operation is used. The above list is sorted from least to
  most specific.

- If both the keysyms and the matching operations are the same (but the
  modifiers are different), the first interpret is used.

As described above, once an interpret "attaches" to a level, it can bind
an action to that level, add one virtual modifier to the key's vmodmap,
or set the key's repeat setting. You should note the following:

- The key repeat is a property of the entire key; it is not level-specific.
  In order to avoid confusion, it is only inspected for the first level of
  the first group; the interpret's repeat setting is ignored when applied
  to other levels.

- If one of the above fields was set directly for a key in xkb_symbols,
  the explicit setting takes precedence over the interpret.

The body of the statement may include statements of the following
forms (all of which are optional):

- useModMapMods statement:

        useModMapMods = level1;

  When set to 'level1', the interpret will only match levels which are
  the first level of the first group of the keys. This can be useful in
  conjunction with e.g. a virtualModifier statement.

- action statement:

        action = LockMods(modifiers=NumLock);

  Bind this action to the matching levels.

- virtual modifier statement:

        virtualModifier = NumLock;

  Add this virtual modifier to the key's vmodmap. The given virtual
  modifier must be declared at the top level of the file with a
  virtual_modifiers statement, e.g.:

        virtual_modifiers NumLock;

- repeat statement:

        repeat = True;

  Set whether the key should repeat or not. Must be a boolean value.

LED map statements
------------------
Statements of the form:

    indicator "Shift Lock" { ... }

This statement specifies the behavior and binding of the LED (a.k.a
indicator) with the given name ("Shift Lock" above). The name should
have been declared previously in the xkb_keycodes section (see LED
name statement), and given an index there. If it wasn't, it is created
with the next free index.
The body of the statement describes the conditions of the keyboard
state which will cause the LED to be lit. It may include the following
statements:

- modifiers statement:

        modifiers = ScrollLock;

  If the given modifiers are in the required state (see below), the
  LED is lit.

- whichModifierState statment:

        whichModState = Latched+Locked;

  Can be any combination of:

  * base, latched, locked, effective
  * any (i.e. all of the above)
  * none (i.e. none of the above)
  * compat (legacy value, treated as effective)

  This will cause the respective portion of the modifer state (see
  struct xkb_state) to be matched against the modifiers given in the
  "modifiers" statement.

  Here's a simple example:

    indicator "Num Lock" {
        modifiers = NumLock;
        whichModState = Locked;
    };

  Whenever the NumLock modifier is locked, the Num Lock LED will light
  up.

- groups statment:

        groups = All - group1;

  If the given groups are in the required state (see below), the LED
  is lit.

- whichGroupState statment:

        whichGroupState = Effective;

  Can be any combination of:

  * base, latched, locked, effective
  * any (i.e. all of the above)
  * none (i.e. none of the above)

  This will cause the respective portion of the group state (see
  struct xkb_state) to be matched against the groups given in the
  "groups" statement.

  Note: the above conditions are disjunctive, i.e. if any of them are
  satisfied the LED is lit.


The xkb_symbols section
=======================

This section is the fourth to be processed, after xkb_keycodes,
xkb_types and xkb_compat.

TODO


Virtual modifier statements
===========================

Statements of the form:
    virtual_modifiers LControl;

Can appear in the xkb_types, xkb_compat, xkb_symbols sections.
TODO