anchor.diff:      (values 0.0)
       (1+ (log (float (/ num-docs (1+ doc-freq)))))))
   
  -(defmethod coord ((self default-similarity) overlap max-overlap)
  +(defun default-coord-function (similarity overlap max-overlap)
  +  (declare (ignore similarity))
     (float (/ overlap max-overlap)))
   
   
  
positive.code:       (incf idf (idf-term self term searcher))) 
      (values idf)))
  
  ;;; ---------------------------------------------------------------------------
  ;;; default-similarity
  ;;; ---------------------------------------------------------------------------
  
  (defclass default-similarity (similarity)
    ())
  
  (defmethod length-norm ((self default-similarity) field num-terms)
    (declare (ignore field))
  
negative.code: 
  (defclass similarity ()
    ((default-similarity-class :allocation :class :initform 'default-similarity
       :accessor default-similarity-class)
     (coord-function :accessor coord-function :initarg :coord-function))
    (:default-initargs
      :coord-function 'coord-function-not-implemented))
  
  (defun make-default-similarity ()
    (make-instance
  