proposition: Lemma validxx f : valid (f \+ f) -> dom f =i pred0.
proof: Proof. by case: validUn=>// _ _ L _ z; case: (_ \in _) (L z)=>//; elim. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)