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