anchor.diff:  ;;; symbol macro entries.
   
   (defclass simple-environment ()
  -  (;; The package namespace.  A list of packages.
  +  (;; A value used in this environment for unbound variables. 
  +   (%unbound :initform (list nil) :reader unbound)
  +   ;; The package namespace.  A list of packages.
      (%packages :initform '() :accessor packages)
      ;; The class namespace.  A list of classes.
      (%classes :initform '() :accessor classes)
  
positive.code:    (%specialp :initform nil :accessor specialp)
     ;; This slot contains the value associated with the variable.  If
     ;; there is no value associated with the variable, then this slot
     ;; contains the value of the variable +UNBOUND+.
     (%value :initform +unbound+ :accessor value)
     ;; The value of this slot is either a macro function, or NIL if
     ;; this entry does not currently represent a symbol macro.
     (%macro-function :initform nil :accessor macro-function)
  
negative.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; Global environment.
  ;;;
  ;;; The global environment is divided into namespaces, but a namespace
  