Section feq.
Variables A B:Type.
Definition feq (f g: A -> B):=forall a, (f a)=(g a).
Infix "=f":= feq  (at level 80, right associativity).
Hint Unfold feq.
intuition.
Qed.
intuition.
Qed.
unfold feq. intuition.
rewrite H.
auto.
Qed.
End feq.
Infix "=f":= feq  (at level 80, right associativity).
Hint Unfold feq. Hint Resolve feq_refl feq_sym feq_trans.
Variable K:(nat -> nat)->Prop.
Variable K_ext:forall a b, (K a)->(a =f b)->(K b).
Add Parametric Relation (A B : Type) : (A -> B) (@feq A B)
reflexivity proved by (@feq_refl A B)
symmetry proved by (@feq_sym A B)
transitivity proved by (@feq_trans A B) as funsetoid.
Add Morphism K with signature (@feq nat nat) ==> iff as K_ext1.
intuition. apply (K_ext H0 H).
intuition. assert (y =f x);auto.  apply (K_ext H0 H1).
Qed.

Lemma three:forall n, forall a, (K a)->(a =f (fun m => (a (n+m))))-> (K (fun m
=> (a (n+m)))).