def xml_for(cls, parts):
        """
        Return content types XML mapping each part in *parts* to the
        appropriate content type and suitable for storage as
        ``[Content_Types].xml`` in an OPC package.
        """
        cti = cls()
        cti._defaults['rels'] = CT.OPC_RELATIONSHIPS
        cti._defaults['xml'] = CT.XML
        for part in parts:
            cti._add_content_type(part.partname, part.content_type)
        return cti._xml()