proposition: Lemma cexit2 h1 h : h1 = empty -> h1 :+ h = h.
proof: Proof. by move=>->; rewrite un0h. Qed.
imports: mathcomp(ssreflect ssrfun ssrnat div ssrbool seq) LemmaOverloading(prelude finmap ordtype) mathcomp(path eqtype) Eqdep