proposition: Lemma joinC x y : pjoin x y = pjoin y x.
proof: Proof. by rewrite /= (joinC x.1) (joinC x.2). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)