Section Fold_Left_Recursor.
Variables (A : Type) (B : Type).
Variable f : A -> B -> A.
Fixpoint fold_left (l:list B) (a0:A) : A :=
match l with
| nil => a0
| cons b t => fold_left t (f a0 b)
end.

Lemma fold_left_app : forall (l l':list B)(i:A),
fold_left (l++l') i = fold_left l' (fold_left l i).