anchor.diff:  (defgeneric reference (object space transformation)
     (:documentation
      "Return a -- potentially optimized and simplified -- data structure
  -   equivalent to an instance of class REFERENCE.")
  +    equivalent to an instance of class REFERENCE.")
     (:method-combination or)
     (:method or ((object data-structure)
                  (space index-space)
  
positive.code: (define-class virtual-machine () ()
    (:documentation
     "A virtual machine is an abstraction over a set of hardware
     resources. All handling of kernels --- such as performance analysis,
     compilation and execution --- is done in the context of a particular
     virtual machine."))
  
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  
negative.code:    (refcount     :type non-negative-fixnum :initform 0 :accessor refcount))
    (:documentation
     "A data structure of dimension D is a mapping from elements of
      INDEX-SPACE to values of type ELEMENT-TYPE.
  
      INPUTS is a list of data structures on which the definition of this data
      structure depends on.
  
      REFCOUNT is an implementation detail. For ordinary data structures it
      tracks how many times the data structure appears as an input of another
      data structure. For immediate data structures, it tracks how many times
      the data structure appears as the source of a kernel."))
  
  (defmethod initialize-instance :after ; reference counting
      ((instance data-structure) &key &allow-other-keys)
  