[GRLUG] xmodmap modifier limit

[Roberto Villarreal]

I just got this new keyboard, and the right Ctrl key is tiny and out in 
no-mans land, so I decided to learn how to modify a keymap.  However:

rvillarreal at fubar:~$ xmodmap -pm
xmodmap:  up to 3 keys per modifier, (keycodes in parentheses):

And it's not lying.  Anyone know if this is an arbitrary limit, and if so, how 
one can change it?  I'd like four... reason being that there are two keys 
that lie where the Ctrl key was on my old keyboard, Super_R and Menu, so I'd 
like to make both of those Ctrl keys, but not at the expense of the 'real' 
Ctrl keys.

Roberto