proposition: Lemma sle_sortedE ks x y : sorted ord ks -> x \in ks -> y \in ks -> x <=[ks] y = oleq x y.
proof: Proof. by move=>S X Y; rewrite oleqNord sleNgt (slt_sortedE S Y X). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)