Cube root by newton's method

cbrt-newtons-method.lisp

(defun my-cube-rt (x)
  (labels (
           (try (guess)
             (let ((improved (improve guess)))
               (if (good-enough? guess improved)
                   improved
                   (try improved))))
           (improve (guess)
             (/ (+ (/ x (square guess)) (* 2 guess)) 3))
           (good-enough? ( guess improved)
             (if (< (abs (- 1 (/ guess improved))) eps )
                 improved
                 nil)))
    (try 1.0)))