(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
  (let ((e1 (make-env))
        (e0 (make-env)))
    (env-vbind e1 'x x1)
    (env-vbind e0 'x x0)
    (- (eval code-fragment e1)
       (eval code-fragment e0))))

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