Create bond block in `CTfile` format.

        :param str key: Ctab atom block key. 
        :return: Ctab bond block.
        :rtype: :py:class:`str`

def _to_ctfile_bond_block(self, key):
        """Create bond block in `CTfile` format.

        :param str key: Ctab atom block key. 
        :return: Ctab bond block.
        :rtype: :py:class:`str`
        """
        counter = OrderedCounter(Bond.bond_block_format)
        ctab_bond_block = '\n'.join([''.join([str(value).rjust(spacing) for value, spacing
                                              in zip(bond._ctab_data.values(), counter.values())])
                                     for bond in self[key]])
        return '{}\n'.format(ctab_bond_block)