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