(define (cddr s)
  (cdr (cdr s)))

(define (cadr s)
  'YOUR-CODE-HERE
)

(define (caddr s)
  'YOUR-CODE-HERE
)


(define (sign num)
  'YOUR-CODE-HERE
)


(define (square x) (* x x))

(define (pow x y)
  'YOUR-CODE-HERE
)