proposition: Lemma In_undef x : x \In (undef : U) -> False.
proof: Proof. by case; 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)