Serialize this relationship collection into XML suitable for storage
        as a .rels file in an OPC package.

def xml(self):
        """
        Serialize this relationship collection into XML suitable for storage
        as a .rels file in an OPC package.
        """
        rels_elm = CT_Relationships.new()
        for rel in self.values():
            rels_elm.add_rel(
                rel.rId, rel.reltype, rel.target_ref, rel.is_external
            )
        return rels_elm.xml