code: def load_rel(self, reltype, target, rId, is_external=False):
          
          return self.rels.add_relationship(reltype, target, rId, is_external)
docs: 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.