xkb setup to turn Caps Lock into Mod3 on Linux (e.c. for use with the i3 window manager)

xkb-keyboard-layout-nh2-thinkpad.md

Followed http://askubuntu.com/questions/482678/how-to-add-a-new-keyboard-layout-custom-keyboard-layout-definition

Add new symbols file

Create /usr/share/X11/xkb/symbols/nh2-thinkpad with content:

// based on a keyboard map from an 'xkb/symbols/gb' file

default  partial alphanumeric_keys
xkb_symbols "basic" {

    // Makes Caps Lock become the "Hyper" key, and be the Mod3 modifier.
    // This allows me to easily use Mod3 in the i3 window manager without
    // messing with `setxkbmap`/`xmodmap`; it even keeps working when
    // re-plugging the keyboard.

    name[Group1]="English (UK) nh2 Thinkpad";
    include "gb"

    key <CAPS>  { [ Hyper_L ] };

    modifier_map Mod3 { <CAPS> };
    modifier_map Mod3 { <HYPR> };

    include "level3(ralt_switch_multikey)"
};

Make symbols available in system-wide choice list

To /usr/share/X11/xkb/rules/evdev.xml add an entire <layout> section, copied from the <layout> section that contains <name>gb</name>, but just below that gb <layout> section. That means, put in:

    <layout>
      <configItem>
        <name>gb-nh2-thinkpad</name>
        <shortDescription>en</shortDescription>
        <description>English (UK) nh2 Thinkpad</description>
        <languageList>
          <iso639Id>eng</iso639Id>
        </languageList>
      </configItem>
      <variantList>
      </variantList>
    </layout>

Then log out and log in again.

The keyboard layout should now be available in the Unity/Gnome/whatever layout selection menu. It even appears in the GDM login screen!