code: def add_title(self, title, subtitle=None, source=None):
          
          title_entry = self._sourced_dict(
              source,
              title=title,
          )
          if subtitle is not None:
              title_entry['subtitle'] = subtitle
  
          self._append_to('titles', title_entry)
docs: Add title.
  
          :param title: title for the current document
          :type title: string
  
          :param subtitle: subtitle for the current document
          :type subtitle: string
  
          :param source: source for the given title
          :type source: string