proposition: Lemma undef0 : (undef : U) <> (Unit : U).
proof: Proof. by move=>E; move: (@valid_unit U); rewrite -E valid_undef. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)