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 key in self:
            if key == 'HeaderBlock':
                for line in self[key].values():
                    output.write(line)
                    output.write('\n')

            elif key == 'Ctab':
                ctab_str = self[key]._to_ctfile()
                output.write(ctab_str)

            else:
                raise KeyError('Molfile object does not supposed to have any other information: "{}".'.format(key))

        return output.getvalue()