anchor.diff:      (check-type description string)
       (check-type short       (or null character))
       (check-type long        (or null string))
  -    (check-type arg-parser  function)
  +    (check-type arg-parser  (or null function))
       (check-type meta-var    string)
       (push (make-instance 'option
                            :name        name
  
positive.code:         (description (getf args :description "?"))
          (short       (getf args :short))
          (long        (getf args :long))
          (arg-parser  (getf args :arg-parser #'identity))
          (meta-var    (getf args :meta-var "ARG")))
      (unless (or short long)
        (error "at least one form of the option must be provided"))
  
negative.code: printed in option description."
    `(progn
       ,@(mapcar (lambda (args) (cons 'add-option args))
                 descriptions)
       nil))
  
  (defun argv ()
    "Return list of program's arguments, including command used to execute the
  