proposition: Lemma uvalid_undef : ~~ valid uundef.
proof: Proof. by rewrite pcmE /valid /= !valid_undef. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)