proposition: Lemma emptyE : empty = to (UM.empty C V) :> U.
proof: Proof. by case: U=>x [*]. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)