A nice explanation on how to define a compose key on a Linux box:
I also came across a cool way to type any character of which you know the hexadecimal value in GTK programs. Type
ctrl+shift+u, enter the hexadecimal value, and enter.
I looks like the method (g)vim uses:
ctrl+v u [hex].
Why would anyone have a need for that? Well, I sometimes use it to input a rare Chinese graph which my Japanese IME doesn’t know about.