proposition: Lemma sle_cons x y k ks : x <=[k :: ks] y = (x == k) || (y != k) && x <=[ks] y.
proof: Proof. by rewrite sleNgt slt_cons negb_and negbK negb_or -sleNgt. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)