/**
  * Return the default Similarity implementation used by indexing and search
  * code.
  *
  * @return ZSearchSimilarity
  */
 public static function getDefault()
 {
     if (!self::$_defaultImpl instanceof ZSearchSimilarity) {
         self::$_defaultImpl = new ZSearchDefaultSimilarity();
     }
     return self::$_defaultImpl;
 }