anchor.diff:  ;;; Datum class SPECIAL-LOCATION.
   ;;;
   ;;; This datum corresponds to a reference to a special variable.
  -;;; The storage is the place where a global value (if any) of the
  -;;; special variable is stored.
  -;;;
  -;;; In later compilation stages, this datum is eliminated.  Instead
  -;;; an EXTERNAL-INPUT is used to hold the storage cell containing the
  -;;; global value, and this cell (together with the name) is passed as
  -;;; argument to a function call that searches for bindings in the
  -;;; dynamic environment.
   
   (defclass special-location (datum)
     ((%name :initarg :name :reader name)))
  
positive.code: ;;;
  ;;; Instruction ENTER-INSTRUCTION.
  ;;;
  ;;; This instruction is mostly a placeholder for backend to put
  ;;; information that needs to be present right at the beginning of
  ;;; each procedure.  It has a single successor.
  
  (defclass enter-instruction (instruction)
    ())
  
  (defun make-enter-instruction (&optional (successor nil successor-p))
    (make-instance 'enter-instruction
      :successors (if successor-p (list successor) '())))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  
negative.code: ;;;
  ;;; This datum corresponds to a reference to a global FUNCTION,
  ;;; i.e., a function defined in the global environment.  
  
  (defclass global-input (datum)
    ((%name :initarg :name :reader name)))
  
  (defun make-global-input (name)
    (make-instance 'global-input
      :name name))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  