Fixpoint sum_leaves (t : btree A) : nat :=
  match t with
  | leaf n => f n
  | node t1 t2 => sum_leaves t1 + sum_leaves t2
  end.Definition sum_order x y := sum_leaves x <= sum_leaves y.Definition le_sum x y := le_bool (sum_leaves x) (sum_leaves y).

Theorem le_sum_correct2 : forall a b1 : btree A, le_sum a b1 = false -> sum_order b1 a.