Compose last characters.
The argument is a parameterized event of the form
(compose-last-chars N COMPONENTS),
where N is the number of characters before point to compose,
COMPONENTS, if non-nil, is the same as the argument to `compose-region'
(which see). If it is nil, `compose-chars-after' is called,
and that function finds a proper rule to compose the target characters.
This function is intended to be used from input methods.
The global keymap binds special event `compose-last-chars' to this
function. Input method may generate an event (compose-last-chars N COMPONENTS)
after a sequence of character events.