proposition: Lemma invalidE f : ~~ valid f <-> f = undef.
proof: Proof. by rewrite /undef !pcmE /= !umEX -2![f]UMC.tfE !umEX; case: (UMC.from f). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)