defgeneric optional-arg-form (input &key gensym-name))
(defmethod optional-arg-form ((input (eql nil)) &key (gensym-name "G"))
  `(,(gensym gensym-name) t))
(defmethod optional-arg-form ((input symbol) &key (gensym-name "G"))
  `(,(gensym gensym-name) (eql ,input)))
(defmethod optional-arg-form ((input list) &key (gensym-name "G"))
  (declare (ignore gensym-name))
  input)

(defmacro define-key-down-event ((key &optional state &rest keys) &body body)
  `(defmethod handle-key-down-event
       (,(optional-arg-form key :gensym-name "KEY-")
        ,(optional-arg-form state :gensym-name "STATE-") &key ,@keys)
     ,@body))