proposition: Lemma umpfx0 h : um_prefix Unit h.
proof: Proof. by rewrite /um_prefix assocs0; apply/prefixE; exists (assocs h). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)