(define (equal? a b)
  (if (nil? a) (if (nil? b) 1 0)
    (if (atom a)
      (if (atom b) (= a b) 0)
      (if (atom b) 0 (if (equal? (car a) (car b)) (equal? (cdr a) (cdr b)) 0))
    )
  )
)

(define (empty? l)
    (equal? l (nil))
)

(define (comb-impl list cur n)
  (if (empty? list)
      (if (= cur n) 1 0)
      (+
       (comb-impl (cdr list) cur n)
       (comb-impl (cdr list) (+ cur (car list)) n)
       )
      )
)

(define (comb list n)
  (comb-impl list 0 n))

(comb (quote(1)) 1)
(comb (quote(1 1)) 1)
(comb (quote(1 1)) 2)
(comb (quote(1 1 1 2)) 3)
(comb (quote(1 2 1 2)) 3)
(comb (quote(1 3 4 2)) 3)
(comb (quote(1 2 3 4 5)) 15)
(comb (quote(1 2 3 4 5)) 10)
(comb (quote(1 2 3 4 5)) 5)