proposition: Lemma sle_rcons_in x y k ks : x \in ks -> x <=[rcons ks k] y = x <=[ks] y.
proof: Proof. by move=>X; rewrite sle_rcons X. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)