End OCCUR.
Section APPEND.
Variable data : Set.
Fixpoint append (q : queue data) : queue data -> queue data :=
fun q2 : queue data =>
match q with
| empty => q2
| input d q' => input data d (append q' q2)
end.

Lemma append_right_non_empty :
forall (q1 q2 : queue data) (d : data),
append q1 (input data d q2) <> empty data.