anchor.diff:  
   (define-presentation-method presentation-typep (object (type symbol))
     (symbolp object))
  +
  +(define-presentation-method presentation-typep (object (type (eql t)))
  +  (declare (ignore object))
  +  t)
  +
  +(define-presentation-method presentation-typep (object (type (eql nil)))
  +  (declare (ignore object))
  +  nil)
   
   (define-presentation-method present (object (type symbol) stream
                                        (view textual-view)
  
positive.code: ;;; The presentation type for T is the built-in type T. The correspondence is
  ;;; established by hand in presentations.lisp.
  #+ (or) (define-presentation-type t ())
  
  ;;; AUTO-ACTIVATE is described in the Franz user guide; it controls whether an
  ;;; accepting an expression returns immediately after typing the closing
  
negative.code:                   :subform-read ,subform-read ))
  
  ;;; The presentation types
  
  (define-presentation-type null ()
    :inherit-from t)
  