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