proposition: Lemma sle_head x ks y : (head x ks) <=[ks] y.
proof: Proof. by case: ks=>[|k ks] //=; rewrite sleL. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)