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