SICP 2.11

(define (+? x) (>= x 0)) ;;; Is x a positive number?
(define (-? x) (< x 0)) ;;; Is x a negative number?

(define (make-interval a b) (cons a b))
(define (lower-bound i) (car i))
(define (upper-bound i) (cdr i))

(define (mul-interval x y)
  (let ((a (lower-bound x))
        (b (upper-bound x))
        (c (lower-bound y))
        (d (upper-bound y)))
    (cond ((and (+? a) (+? b) (+? c) (+? d)) (make-interval (* a c) (* b d)))
          ((and (+? a) (+? b) (-? c) (+? d)) (make-interval (* b c) (* b d)))
          ((and (+? a) (+? b) (-? c) (-? d)) (make-interval (* b c) (* a d)))
          ((and (-? a) (+? b) (+? c) (+? d)) (make-interval (* a d) (* b d)))
          ((and (-? a) (+? b) (-? c) (+? d))
           (let ((p1 (min (* a d) (* b c)))
                 (p2 (max (* a c) (* b d))))
             (make-interval p1 p2)))
          ((and (-? a) (+? b) (-? c) (-? d)) (make-interval (* b c) (* a c)))
          ((and (-? a) (-? b) (+? c) (+? d)) (make-interval (* a d) (* b c)))
          ((and (-? a) (-? b) (-? c) (+? d)) (make-interval (* a d) (* a c)))
          ((and (-? a) (-? b) (-? c) (-? d)) (make-interval (* b d) (* a c))))))