(mbe-defrules my-incf
  ;; 引数が1つのときは1足す
  ((var) (setq var (+ var 1)))
  ;; 引数が2つのときはby足す
  ((var by) (setq var (+ var by))))
(let ((a 0) (b 0))
  (incf a)                              ; => 1
  (incf a 10)                           ; => 11
  a                                     ; => 11
  (my-incf b)                           ; => 1
  (my-incf b 10)                        ; => 11
  b                                     ; => 11
  )