(mod (X)
  (include *strict-cl-21*)

  ;; A macro-level function to pass through only real integers.
  (defun pass-through-integers (X)
    (if (not (number? X))
      (x "not a number given to only-integers" X)
      X
      )
    )

  ;; A macro which at preprocessing time throws if the given argument
  ;; wasn't a lexical integer.
  (defmac only-integers (X) (pass-through-integers X))

  ;; Note: when macro expanding, N is the N argument to the body of
  ;; the double macro, not the integer literal, so we use the function
  ;; version of pass-through-integers in the macro body.
  (defmac double (N) (* 2 (pass-through-integers N)))

  ;; Here the macro form of only-integers can determine whether the
  ;; double macro produced an integer or some other expression.
  (defun F (N) (+ N (only-integers (double 99))))

  (F X)
  )