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

Lemma fold_right_app : forall (A B:Type)(f:A->B->B) l l' i,
fold_right f i (l++l') = fold_right f (fold_right f i l') l.