proposition: Lemma sle_rcons x y k ks : x <=[rcons ks k] y = if x \in ks then x <=[ks] y else (y \notin ks) && ((k == x) || (k != y)).
proof: Proof. by rewrite !sleNgt slt_rcons; case: ifP=>//; rewrite negb_or negb_and negbK. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)