proposition: Lemma sle_memI x y ks : y \notin ks -> x <=[ks] y.
proof: Proof. by move/index_memN=>E; rewrite /seq_le E index_size. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)