anchor.diff:  ;;; 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 ())
  +(define-presentation-type nil ())
   
   ;;; AUTO-ACTIVATE is described in the Franz user guide; it controls whether an
   ;;; accepting an expression returns immediately after typing the closing
  
positive.code:                   :subform-read ,subform-read ))
  
  ;;; The presentation types
  
  (define-presentation-method presentation-typep (object (type t))
    (declare (ignore object))
    t)
  
  (define-presentation-method present (object (type t)
                                              stream
                                              (view textual-view)
                                              &key acceptably for-context-type)
    (declare (ignore for-context-type))
    (let ((*print-readably* acceptably))
      (if acceptably
          (prin1 object stream)
          (princ object stream))))
  
  (define-presentation-type nil ())
  
  (define-presentation-method presentation-typep (object (type nil))
    (declare (ignore object))
    nil)
  
  (define-presentation-type null ()
    :inherit-from t)
  
negative.code: 
  (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)
  