anchor.diff:    (:method ((grammar grammar) non-terminal)
       (follow-set (grammar-normalized-grammar grammar) non-terminal))
     (:method ((grammar normalized-grammar) non-terminal)
  -    (or (gethash non-terminal (grammar-follow-sets grammar))
  -        (error "~S is not a non-terminal of the grammar ~A"
  -               non-terminal (grammar-name grammar)))))
  +    (multiple-value-bind (follow-set presentp)
  +        (gethash non-terminal (grammar-follow-sets grammar))
  +      (if presentp
  +          follow-set
  +          (error "~S is not a non-terminal of the grammar ~A"
  +                 non-terminal (grammar-name grammar))))))
   
   
   (defgeneric rhs-firsts-set (grammar non-terminal rhs)
  
positive.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;; THE END ;;;;
  
  
negative.code:       (setf (normalized-grammar-%synthesized-non-terminals new-grammar)
              (set-difference (grammar-all-non-terminals new-grammar)
                              original-non-terminals)))
      (compute-first-follow  new-grammar)
      new-grammar))
  
  (defgeneric normalize-grammar (grammar)
    (:documentation "Return a new normalized grammar parsing the same language as GRAMMAR.")
  