add-completion-to-tail-if-new is a compiled Lisp function in `completion.el'.

(add-completion-to-tail-if-new STRING)

If STRING is not in the database add it to appropriate prefix list.
STRING is added to the end of the appropriate prefix list with
num-uses = 0. The database is unchanged if it is there. STRING must be
longer than `completion-prefix-min-length'.
This must be very fast.
Returns the completion entry.