completion-try-completion is a compiled Lisp function in `minibuffer.el'.

(completion-try-completion STRING TABLE PRED POINT &optional METADATA)

Try to complete STRING using completion table TABLE.
Only the elements of table that satisfy predicate PRED are considered.
POINT is the position of point within STRING.
The return value can be either nil to indicate that there is no completion,
t to indicate that STRING is the only possible completion,
or a pair (NEWSTRING . NEWPOINT) of the completed result string together with
a new position for point.