docs: def _element(self):
          """
          Return XML form of this content types item, suitable for storage as
          ``[Content_Types].xml`` in an OPC package. Although the sequence of
          elements is not strictly significant, as an aid to testing and
          readability Default elements are sorted by extension and Override
          elements are sorted by partname.
          """
          _types_elm = CT_Types.new()
          for ext in sorted(self._defaults.keys()):
              _types_elm.add_default(ext, self._defaults[ext])
          for partname in sorted(self._overrides.keys()):
              _types_elm.add_override(partname, self._overrides[partname])
          return _types_elm