proposition: Lemma last_notin x k (s : seq A) : x \in s -> k \notin s -> last k s != k.
proof: Proof. by move/(last_in _ (k:=k))=>H /negbTE; case: eqP H=>// ->->. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat seq path eqtype choice bigop) pcm(options prelude pred)