anchor.diff:    (let ((collection (make-instance 'document-collection)))
       (loop for sentence in (sentence-tokenize text)
             do (add-document collection
  -                           (make-instance 'document-similarity-vector
  +                           (make-instance 'document-vertex
                                             :string-contents sentence)))
       (word-count-vectorize-documents collection)
       (generate-document-similarity-vectors collection)
  
positive.code:                (remove document documents))
               (graph-neighbor-edge-sum (document)
                 "Add up the edges of all neighbors of a given node."
                 (let ((sum (- (reduce #'+ (mapcar #'car (similarity-vector document))) 1)))
                   (if (> sum 0) sum 1)))
               (document-similarity (document-a document-b)
                 (car (find document-b (similarity-vector document-a) :key #'cdr)))
               (convergedp (previous-score current-score)
                 "Check if a delta qualifies for convergence."
                 (<=  (abs (- previous-score current-score)) epsilon)))
  
negative.code: 
  ;;; text-rank.lisp -- implementation of textrank algorithm
  
  (defclass document-vertex (document)
    ((edges :accessor edges :initform (make-hash-table)
            :documentation "The keys of the hash table represent the
            edges, the values of the hash table represent the edge
            weights.")))
  
  (defmethod word-count-vectorize ((document document) dictionary)
    "Transform a document into a vector using word counts."
  