proposition: Lemma sexit1 h1 h2 h : def (h2 :+ h) -> (def h2 -> supdom h2 h1) -> supdom (h2 :+ h) (h :+ h1).
proof: Proof. move=>H1 H2; rewrite (unC h); apply: subdomE=>//. by apply: H2; apply: defUnl H1. Qed.
imports: mathcomp(ssreflect ssrfun ssrnat div ssrbool seq) LemmaOverloading(prelude finmap ordtype) mathcomp(path eqtype) Eqdep