proposition: Lemma slt_cat ks1 ks2 x y : x <[ks1++ks2] y = if y \in ks1 then x <[ks1] y else (x \in ks1) || x <[ks2] y.
proof: Proof. by rewrite !sltNge sle_cat; case: ifP=>//; rewrite negb_and negbK. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)