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	ITERATED SUMS
*)

Fixpoint sum_nm (n : nat) : nat -> (nat -> nat) -> nat :=
  fun (m : nat) (f : nat -> nat) =>
  match n with
  | O => f m
  | S n' => f m + sum_nm n' (S m) f
  end.

Lemma sum_nm_i : forall (m n : nat) (f : nat -> nat), sum_nm (S n) m f = f m + sum_nm n (S m) f.