infix.scm

;;; SICP 2.58 part.b
;;;
;; sum
(define (make-sum a1 a2)
  (cond ((and (number? a1) (number? a2)) (+ a1 a2))
        ((=number? a1 0) a2)
	((=number? a2 0) a1)
	(else (list  a1 '+ a2))))
(define (sum? x)
  (and (pair? x) (eq? (cadr x) '+)))
(define (addend s)
  (car s))
(define (augend s)
  (if (null? (cdddr s))
      (caddr s)
      (cddr s)))
;; product
(define (make-product m1 m2)
  (cond 
   ((or (=number? m1 0) (=number? m2 0)) 0)
   ((and (number? m1) (number? m2)) (* m1 m2))
   ((=number? m1 1) m2)
   ((=number? m2 1) m1)
   (else (list  m1 '* m2))))

(define (product? x)
  (and (pair? x) (eq? (cadr x) '*)))

(define (multiplier e)
  (car e))
(define (multiplicand p)
  (if (null? (cdddr p))
      (caddr p)
      (cddr p)))