linux - Xorg compose key without losing valuable key / bypass mode?

I would like to setup a compose key, but the only options available in the KDE system settings (and via /usr/share/X11/xkb/rules) are:

compose:ralt         Right Alt
compose:lwin         Left Win
compose:rwin         Right Win
compose:menu         Menu
compose:lctrl        Left Ctrl
compose:rctrl        Right Ctrl
compose:caps         Caps Lock
compose:102          Less-than/Greater-than

None of those please me though; "compose:caps" might be ok, but it's mapped to ctrl already.

Is there an option so setup some key (e.g. some of those extra function keys), or to get the key itself somehow? (e.g. ralt would be OK, if I could easily get the original behavior of the key (AltGr))

I would love to have something like Shift-AltGr to trigger Compose mode.

I guess I could add an extra rule, like it's done for/with compose:102.. what does 102 refer to? (it's not something dumped via xev).

There is also an option lv3:ralt_switch_multikey (Right Alt, Shift+Right Alt key is Multi_Key), but this did not work as expected (see http://bugs.kde.org/246472).

Answer

lv3:ralt_switch_multikey is what I was looking for:

# But here it is: /usr/share/X11/xkb/rules/xorg.lst
# Anyway... what this is doing:
# lvl3:ralt_switch_multikey
#   right alt = AltGr = choose third shift level
#   shift + right alt = compose key aka Multi_Key

(via http://ndim.fedorapeople.org/stuff/radeonhd/xorg.conf)

I have this set via the KDE system settings (Keyboard => Advanced), so that I have the following in my ~/.kde/share/config/kxkbrc:

Options=ctrl:nocaps,lv3:ralt_switch_multikey