数値積分のシンプソンの公式

SICPの問題1.29です。

(define (sum term a next b)
  (if (> a b)
      0
      (+ (term a)
	 (sum term (next a) next b))))

(define (integral f a b n)
  (let* ((h (/ (- b a) n))
	(inc (lambda (x) (+ x 1)))
	(term (lambda (k)
		(cond [(or (= k 0) (= k n)) (f (+ a (* k h)))]
		      [(odd? k) (* 4 (f (+ a (* k h))))]
		      [else (* 2 (f (+ a (* k h))))]))))
    (* (sum term 0 inc n) (/ h 3.0))))