anchor.diff:     describes how elements of the storage of TARGET can be computed by using
      elements of the storage of SOURCES. ITERATION-SPACE is a subspace of the
      index space of the storage of TARGET."))
  +
  +(define-class execution-context ()
  +  (:documentation
  +   "An execution context is an abstraction over a set of hardware
  +   resources. All handling of kernels --- such as performance analysis,
  +   compilation and execution --- is done in a particular execution
  +   context."))
   
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;;;
  
positive.code: 
  (defgeneric subspace? (space-1 space-2)
    (:documentation
     "Return true if every index in SPACE-1 occurs also in SPACE-2.")
    (:method ((space-1 t) (space-2 t))
      (equal? space-1 (intersection space-1 space-2))))
  
  
negative.code:    "A data structure of dimension D is a mapping from elements of
     INDEX-SPACE to values of type ELEMENT-TYPE."))
  
  (defmethod initialize-instance :after ; reference counting
      ((instance data-structure) &key &allow-other-keys)
    (map nil (λ input (incf (refcount input))) (inputs instance)))
  
  (define-class immediate (data-structure)
  