proposition: Lemma joinC (x y : U) : x \+ y = y \+ x.
proof: Proof. by rewrite /join; case: U x y=>tp [v j z Cj *]; apply: Cj. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)