anchor.diff:                               (symbol parameter)
                                ((cons symbol null)
                                 (car parameter)))))
  +
  +(defmethod unparse ((parameter optional-no-defaulting-parameter))
  +  (variable parameter))
   
   (defclass parameter-keyword-name-mixin ()
     ((%keyword-name :initarg :keyword-name
  
positive.code:     (make-instance 'optional-parameter
                     :variable variable
                     :initform initform)))
  
negative.code:   (check-type parameter symbol)
    (make-instance 'simple-parameter :variable parameter))
  
  (defmethod unparse ((parameter simple-parameter))
    (variable parameter))
  
  (defclass required-parameter (parameter parameter-variable-mixin)
    ())
  
  