code: def _to_ctfile_bond_block(self, key):
          
          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)
docs: Create bond block in `CTfile` format.
  
          :param str key: Ctab atom block key. 
          :return: Ctab bond block.
          :rtype: :py:class:`str`