docs: def default(self, o):
          """Default encoder.
  
          :param o: Atom or Bond instance.
          :type o: :class:`~ctfile.ctfile.Atom` or :class:`~ctfile.ctfile.Bond`.
          :return: Dictionary that contains information required for atom and bond block of ``Ctab``.
          :rtype: :py:class:`collections.OrderedDict`
          """
          if isinstance(o, Atom) or isinstance(o, Bond):
              return o._ctab_data
          else:
              return o.__dict__