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