anchor.diff:    (vectorize-documents document-collection #'word-count-vectorize))
   
   (defmethod tf-vectorize-documents ((document-collection document-collection))
  +  "Definition: tf: term frequency. How often does a term appear in a
  +document?"
     (vectorize-documents document-collection #'tf-vectorize))
   
   (defmethod tf-idf-vectorize-documents ((document-collection document-collection))
  +  "Definition: tf-idf: term frequency, inverse document frequency. How
  +often does a term appear in a document as compared to all other
  +documents?"
     (vectorize-documents document-collection (lambda (document dictionary)
                                                (tf-idf-vectorize document document-collection dictionary))))
   
  
positive.code:     (setf (vector-data document) vector-data)))
  
  (defmethod tf-vectorize ((document document) dictionary)
    "Transform a document into a vector using tf."
    (let ((vector-data (make-array (length dictionary) :initial-element 0)))
      (loop for word in dictionary
            for index from 0 below (length vector-data)
  
negative.code:     (setf (vector-data document) vector-data)))
  
  (defmethod tf-idf-vectorize ((document document) (collection document-collection) dictionary)
    "Transform a document into a vector using tf-idf.
  Definition: tf-idf: term frequency, inverse document frequency. How
  often does a term a appear in a document as compared to all other
  documents?"
    (let ((vector-data (make-array (length dictionary) :initial-element 0)))
      (loop for word in dictionary
            for index from 0 below (length vector-data)
  