docs: def _to_ctfile_counts_line(self, key):
          """Create counts line in ``CTfile`` format.
  
          :param str key: Counts line key. 
          :return: Counts line string.
          :rtype: :py:class:`str`
          """
          counter = OrderedCounter(self.counts_line_format)
          self[key]['number_of_atoms'] = str(len(self.atoms))
          self[key]['number_of_bonds'] = str(len(self.bonds))
          counts_line = ''.join([str(value).rjust(spacing) for value, spacing
                                 in zip(self[key].values(), counter.values())])
          return '{}\n'.format(counts_line)