docs: def _to_ctfile(self):
          """Convert :class:`~ctfile.ctfile.CTfile` into `CTfile` formatted string.
  
          :return: ``CTfile`` formatted string.
          :rtype: :py:class:`str`.
          """
          output = io.StringIO()
  
          for entry in self.values():
              output.write(entry['molfile']._to_ctfile())
  
              for header, values in entry['data'].items():
                  output.write('> <{}>\n'.format(header))
                  output.write('\n'.join(values))
                  output.write('\n')
              output.write('\n$$$$\n')
  
          return output.getvalue()