(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
  (let ((e1 (make-env (list (cons 'x x1))))   ;; create two environments stuffed with binding for x
        (e0 (make-env (list (cons 'x x0)))))
    (- (eval code-fragment e1)                ;; pass these environment to eval
       (eval code-fragment e0))))

(eval-subtract-for-two-values-of-x '(exp x) 1 2)