(defun quadratic-roots (a b c)
  "Returns the roots of a quadratic equation aX^2 + bX + c = 0"
  (let ((discriminant (- (* b b) (* 4 a c))))
    (list (/ (+ (- b) (sqrt discriminant)) (* 2 a))
          (/ (- (- b) (sqrt discriminant)) (* 2 a)))))