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

@property
def xml(self):

    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
