code: def add_private_note(self, private_notes, source=None):
          
          self._append_to('_private_notes', self._sourced_dict(
              source,
              value=private_notes,
          ))
docs: Add private notes.
  
          :param private_notes: hidden notes for the current document
          :type private_notes: string
  
          :param source: source for the given private notes
          :type source: string