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Generalizable Variables T A.
Inductive path `(a: A): A -> Type := idpath: path a a.
Class TMonad (T: Type -> Type) := {
bind: forall {A B: Type}, (T A) -> (A -> T B) -> T B;
ret: forall {A: Type}, A -> T A;
ret_unit_left: forall {A B: Type} (k: A -> T B) (a: A),
path (bind (ret a) k) (k a)
}.
Definition T_fzip `{TMonad T} := fun (A B: Type) (f: T (A -> B)) (t: T A)
=> bind t (fun a => bind f (fun g => ret (g a) )).
Definition T_pure `{TMonad T} := @ret _ _.
Definition T_pure_id `{TMonad T} {A: Type} (t: A -> A) (x: T A):
path (T_fzip A A (T_pure (A -> A) t) x) x.
unfold T_fzip, T_pure.
Fail rewrite (ret_unit_left (fun g a => ret (g a)) (fun x => x)).
Abort.
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