(in-package :lizfcm.approx)
(defun derivative-at (f x &optional (delta 0.01))
(let* ((x2 (+ x delta))
(x1 (- x delta))
(y2 (apply f (list x2)))
(y1 (apply f (list x1))))
(/ (- y2 y1)
(- x2 x1))))
