proposition: Lemma valid_undefN : ~~ valid (um_undef: U).
proof: Proof. by rewrite pcmE /= !umEX. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)