proposition: Lemma validL (x y : U) : valid (x \+ y) -> valid x.
proof: Proof. rewrite /valid/join. by case: U x y=>tp [v j z Cj Aj Uj /= Mj inv f ?]; apply: Mj. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)