docs: def _to_star(self):
          """Save :class:`~nmrstarlib.nmrstarlib.StarFile` into NMR-STAR or CIF formatted string.
  
          :return: NMR-STAR string.
          :rtype: :py:class:`str`
          """
          star_str = io.StringIO()
          self.print_file(star_str)
          return star_str.getvalue()