The compose key it's defined to do key combinations. It's Used to produce special characters like tildes and accents. To see the list of key names open /usr/share/X11/xkb/rules/xorg.lst and search for "compose".
This technique was tested in Lubuntu. Two ways to enter special characters:
- Call setxkbmap to define compose key in the current session
- Define compose key permanently
1). Command line
$ setxkbmap -option "compose:rwin". Example to define compose key right 'Win'.
2). Open file
/etc/default/keyboard and add line XKBOPTIONS="compose:rwin" (requires restart).
Example to write character ã after defining compose key. Press the compose key, then key 'a' and finally the tilde (AltGr + 4).