proposition: Lemma undefE : um_undef = to (UM.Undef 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)