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