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