anchor.diff:  	  (extract-one-web du-chains)
   	(cons web (chains-to-webs rest)))))
   
  -(defun compute-webs (program)
  +(defun compute-web-sets (program)
     (let ((du-chains (compute-du-chains program))
   	(webs '()))
       (loop until (null du-chains)
  
positive.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; Do some initial transformations.
  
  (defun initial-transformations (program)
    (make program 'no-redundant-temporaries)
    (make program 'no-error-successors)
    (make program 'simplified-instructions)
  
negative.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; Compute web sets.
  ;;;
  ;;; Two DU-chains are said to INTERSECT if and only if:
  ;;;
  