proposition: Lemma JOIN_ret : JoinLaws.left_unit (@CRet M N) (@JOIN').
proof: Proof. move=> A. rewrite [JOIN' _]/=. rewrite JOINE -/prod. by rewrite (Prod.JOIN_ret prod1 prod2). Qed.
imports: mathcomp(all_ssreflect) HB(structures) monae_lib hierarchy monad_lib