proposition: Lemma slt_memE x y ks : x <[ks] y -> x \in ks.
proof: Proof. by rewrite -index_mem=>/leq_trans; apply; rewrite index_size. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)