(define (make-sum a b . rest)
(letrec ((make-sum-sub (lambda (terms sum)
(cond [(zero? sum) (cond [(null? terms) 0]
[(null? (cdr terms))
(car terms)]
[else (cons '+ terms)])]
[(null? terms) sum]
[else (cons '+ (cons sum terms))]))))
(let ((terms (filter
(lambda (x) (not (number? x)))
(cons a (cons b rest))))
(sum (fold + 0
(filter (lambda (x) (number? x))
(cons a (cons b rest))))))
(make-sum-sub terms sum))))
(define (augend s) (if (null? (cdddr s))
(caddr s)
(apply make-sum (cddr s))))
(define (make-product a b . rest)
(letrec ((make-product-sub (lambda (terms product)
(cond [(= product 0) 0]
[(= product 1) (cond [(null? terms) 1]
[(null? (cdr terms))
(car terms)]
[else (cons '* terms)])]
[(null? terms) product]
[else (cons '* (cons product terms))]))))
(let ((terms (filter
(lambda (x) (not (number? x)))
(cons a (cons b rest))))
(product (fold * 1
(filter (lambda (x) (number? x))
(cons a (cons b rest))))))
(make-product-sub terms product))))
(define (multiplicand p) (if (null? (cdddr p))
(caddr p)
(apply make-product (cddr p))))
