docs: def _unmarshal_relationships(pkg_reader, package, parts):
          """
          Add a relationship to the source object corresponding to each of the
          relationships in *pkg_reader* with its target_part set to the actual
          target part in *parts*.
          """
          for source_uri, srel in pkg_reader.iter_srels():
              source = package if source_uri == '/' else parts[source_uri]
              target = (srel.target_ref if srel.is_external
                        else parts[srel.target_partname])
              source.load_rel(srel.reltype, target, srel.rId, srel.is_external)