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