Section fold.
Variables (A : Type) (B : Type).
Variable f : A -> B -> A.
Variable g : B -> A -> A.
Variable h : A -> A.
Variable eqA_dec : forall a b : A, {a = b} + {a <> b}.
Proof using.
intros a l1; generalize a; elim l1; simpl in |- *; auto; clear a l1.
Qed.

Theorem fold_left_eta :
forall l a f1,
(forall a b, In b l -> f a b = f1 a b) -> fold_left f l a = fold_left f1 l a.