proposition: Lemma validR (x y : U) : valid (x \+ y) -> valid y.
proof: Proof. by rewrite joinC; apply: validL. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)