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}.

Theorem fold_left_app :
forall a l1 l2, fold_left f (l1 ++ l2) a = fold_left f l2 (fold_left f l1 a).