(define (cubert x)
  (define (improve guess x)
    (/
     (+ (/ x (* guess guess)) (* 2 guess))
     3)
    )

  (define (cube x)
    (* x x x))
    (define (good-enough? guess x)
    (< (abs (- (cube guess) x)) 0.001))
  (define (cube-iter guess x)
    (if (good-enough? guess x)
        guess
        (cube-iter (improve guess x) x)))
  (cube-iter 1.0 x)
  )