anchor.diff:                        (cons (car x)
                               (/ (cdr x) sum)))
                     v)))
  -    (sort v #'< :key #'car)))
  +    (let ((r (sort v #'< :key #'car)))
  +      (if (eq kind :compacted-binary)
  +          (compact-binary-feature-vector r)
  +          r))))
  
positive.code:                          (incf (cdr (aref v pos)))
                           (vector-push-extend (cons index #.(flt 1)) v))))))
               document)
      (when (member kind '(:binary :normalized-binary))
        (loop for x across v
              do (setf (cdr x) #.(flt 1))))
      (when (member kind '(:normalized-binary :normalized-frequency))
  
negative.code: 
  ;;;; Encoding
  
  (defun compact-binary-feature-vector (feature-vector)
    (make-array (length feature-vector)
                :element-type '(unsigned-byte 32)
                :initial-contents (map 'vector #'car feature-vector)))
  
  (defun encode/bag-of-words (document mapper feature->index &key (kind :binary))
    "Return a sparse vector that represents the encoded DOCUMENT. Get
  the features of DOCUMENT from MAPPER, convert each feature to an index
  