(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
)


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


(define (replicate x n)
  'YOUR-CODE-HERE
  )


(define (accumulate combiner start n term)
  'YOUR-CODE-HERE
)


(define (accumulate-tail combiner start n term)
  'YOUR-CODE-HERE
)


(define-macro (list-of map-expr for var in lst if filter-expr)
  'YOUR-CODE-HERE
)

