Define COMMAND as a prefix command. COMMAND should be a symbol.
A new sparse keymap is stored as COMMAND's function definition and its value.
If a second optional argument MAPVAR is given, the map is stored as
its value instead of as COMMAND's value; but COMMAND is still defined
as a function.
The third optional argument NAME, if given, supplies a menu name
string for the map. This is required to use the keymap as a menu.
This function returns COMMAND.
(fn COMMAND &optional MAPVAR NAME)