Convert :class:`~ctfile.ctfile.CTfile` into `CTfile` formatted string.

        :return: ``CTfile`` formatted string.
        :rtype: :py:class:`str`.

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()