docs: def unmarshal(pkg_reader, package, part_factory):
          """
          Construct graph of parts and realized relationships based on the
          contents of *pkg_reader*, delegating construction of each part to
          *part_factory*. Package relationships are added to *pkg*.
          """
          parts = Unmarshaller._unmarshal_parts(
              pkg_reader, package, part_factory
          )
          Unmarshaller._unmarshal_relationships(pkg_reader, package, parts)
          for part in parts.values():
              part.after_unmarshal()
          package.after_unmarshal()