proposition: Corollary join_ret_x : ∀ a x, join (ret x) = @id _ (m a) x.
proof: Proof. intros. pose proof (@join_ret Coq m M a) as HA. rewrites; reflexivity. Qed.
imports: Category.Lib Category.Theory.Category Category.Theory.Functor Category.Theory.Monad Category.Instance.Coq