proposition: Lemma joinA x y z : pjoin x (pjoin y z) = pjoin (pjoin x y) z.
proof: Proof. by rewrite /= !joinA. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)