proposition: Lemma validL x y : pvalid (pjoin x y) -> pvalid x.
proof: Proof. by case/andP => /= /validL-> /validL->. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)