proposition: Lemma pcm_joinE (x y : U) : x \+ y = join_op (class U) x y.
proof: Proof. by rewrite /join. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)