docs: def _to_json(self, sort_keys=False, indent=4):
          """Convert :class:`~ctfile.ctfile.CTfile` into JSON string.
  
          :return: ``JSON`` formatted string.
          :rtype: :py:class:`str`.
          """
          return json.dumps(self, sort_keys=sort_keys, indent=indent, cls=CtabAtomBondEncoder)