proposition: Lemma memNindexlast x s : x \notin s -> indexlast x s = size s.
proof: Proof. by rewrite -has_pred1=>/hasNfindlast. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat seq path eqtype choice bigop) pcm(options prelude pred)