xmodmap accepts the following input expressions:
pointer = default reset pointer buttons to default
pointer = NUMBER ... set pointer button codes
keycode NUMBER = [KEYSYM ...] map keycode to given keysyms
keysym KEYSYM = [KEYSYM ...] look up keysym and do a keycode operation
clear MODIFIER remove all keys for this modifier
add MODIFIER = KEYSYM ... add the keysyms to the modifier
remove MODIFIER = KEYSYM ... remove the keysyms from the modifier
where NUMBER is a decimal, octal, or hex constant; KEYSYM is a valid
Key Symbol name; and MODIFIER is one of the eight modifier names: Shift,
Lock, Control, Mod1, Mod2, Mod3, Mod4, or Mod5. Lines beginning with
an exclamation mark (!) are taken as comments. Case is significant except
for MODIFIER names.
Keysyms on the left hand side of the = sign are looked up before any changes
are made; keysyms on the right are looked up after all of those on the left
have been resolved. This makes it possible to swap modifier keys.