(defmacro defrule (name lambda-list &rest body)
  (destructuring-bind (input-parameter &rest other-parameters) lambda-list
    `(defun ,(intern (format "parse-%s" name)) ,(cons input-parameter other-parameters)
       (unless (zerop (length ,input-parameter))
         ,@body))))

(macroexpand-1 '(defrule ordinary (input)
                  (list (substring input 1 nil) (list 'literal (aref input 0)))))
--> (defun parse-ordinary (input) (unless (zerop (length input)) (list (substring input 1 nil) (list (quote literal) (aref input 0)))))