proposition: Lemma valid_undefbI f : valid f -> undefb f = false.
proof: Proof. move=>W; apply/negP=>/undefbP E. by move: E W=>->; rewrite valid_undef. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)