proposition: Theorem add_assoc_iso : forall a b c, (a + b) + c ≅ a + (b + c).
proof: Proof. intros. exists (fun p => match p with | inl (inl x) => inl x | inl (inr x) => inr (inl x) | inr x => inr (inr x) end). exists (fun p => match p with | inl x => inl (inl x) | inr (inl x) => inl (inr x) | inr (inr x) => inr x end). extensionalize. Qed.
imports: Hask.Ltac Hask.Prelude Hask.Control.Applicative Hask.Control.Monad Hask.Control.Monad.Cont Hask.Control.Monad.State Hask.Data.Functor.Contravariant Hask.Data.Functor.Identity Hask.Data.Functor.Kan Hask.Data.Functor.Yoneda Hask.Control.Lens Coq.Classes.Morphisms Coq.Setoids.Setoid Coq.Init.Datatypes Coq.Relations.Relation_Definitions FunctionalExtensionality