Return newly added |_Relationship| instance of *reltype* between this
        part and *target* with key *rId*. Target mode is set to
        ``RTM.EXTERNAL`` if *is_external* is |True|. Intended for use during
        load from a serialized package, where the rId is well known. Other
        methods exist for adding a new relationship to the package during
        processing.

def load_rel(self, reltype, target, rId, is_external=False):
        """
        Return newly added |_Relationship| instance of *reltype* between this
        part and *target* with key *rId*. Target mode is set to
        ``RTM.EXTERNAL`` if *is_external* is |True|. Intended for use during
        load from a serialized package, where the rId is well known. Other
        methods exist for adding a new relationship to the package during
        processing.
        """
        return self.rels.add_relationship(reltype, target, rId, is_external)