Construct and return a new sparse keymap.
Its car is `keymap' and its cdr is an alist of (CHAR . DEFINITION),
which binds the character CHAR to DEFINITION, or (SYMBOL . DEFINITION),
which binds the function key or mouse event SYMBOL to DEFINITION.
Initially the alist is nil.
The optional arg STRING supplies a menu name for the keymap
in case you use it as a menu with `x-popup-menu'. (fn &optional STRING)