[tlug] Adding a modifier to a keysym with xmodmap

["Josh Glover" <jmglov@example.com>]

Is it possible to add a modifier to a keysym with xmodmap? I have a
fancy Dell multimedia keyboard with all sorts of crazy extra buttons,
and I would like to turn them into useful accelerators. I have mapped
the keycodes to X keysyms with xmodmap as follows:

: jglover@example.com; cat .xmodmaprc
keycode 115 = Super_L
keycode 116 = Super_R
keycode 117 = Menu
keycode 130 = XF86HomePage
keycode 151 = XF86AudioStop
keycode 159 = XF86AudioPlay
keycode 161 = XF86Calculator
keycode 162 = XF86AudioNext
keycode 164 = XF86AudioPrev
keycode 166 = XF86AudioMute
keycode 196 = XF86AudioRaiseVolume
keycode 199 = XF86MyComputer
keycode 209 = XF86Refresh
keycode 232 = XF86Stop
keycode 233 = XF86Forward
keycode 234 = XF86Back
keycode 236 = XF86Mail

: jglover@example.com; grep xmodmap .xinitrc
xmodmap ${HOME}/.xmodmaprc

What I would really like to do is make the XF86AudioRaiseVolume keysym
generate a "Control + Tab", so that I can use it for, say, switching
tabs in Firefox.

I know how to customise bindings in Firefox, i.e.

http://www.mozilla.org/unix/customizing.html#keys

but it just seems like it would be useful to know how to do this at
the xmodmap level.

Cheers,
Josh