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