'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.'

@property
def _element(self):

    _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
