how would one map a key to a mouse button?
(I have a footswitch that generates
keycode 105 (keysym 0xff56, Next)
in xev when I press it, and I'd like to map that to left-click.)

Web searches seem to indicate that the /etc/X11/X0pointerkeys
had something to do with this on at least old HP systems.

(I'm using Ubuntu Breezy, which has a modern
x.org server.)