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*.

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)