docs: def register_index(self, index):
      """Registers a given index:
  
      * Creates and opens an index for it (if it doesn't exist yet)
      * Sets some default values on it (unless they're already set)
  
      Args:
          index (PonyWhoosh.Index): An instance of PonyWhoosh.Index class
      """
  
      self._indexes[index._name] = index
      self.create_index(index)
      return index