Replace OLDDEF with NEWDEF for any keys in KEYMAP now defined as OLDDEF.
In other words, OLDDEF is replaced with NEWDEF where ever it appears.
Alternatively, if optional fourth argument OLDMAP is specified, we redefine
in KEYMAP as NEWDEF those keys which are defined as OLDDEF in OLDMAP.
If you don't specify OLDMAP, you can usually get the same results
in a cleaner way with command remapping, like this:
(define-key KEYMAP [remap OLDDEF] NEWDEF)
(fn OLDDEF NEWDEF KEYMAP &optional OLDMAP)