x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


(defun eval-with-x (program a b)
  (let ((at-a (eval `(let ((x ',a)) ,program)))
        (at-b (eval `(let ((x ',b)) ,program))))
    (- at-b at-a)))