code: def handle_package_has_file_helper(self, pkg_file):
          
          nodes = list(self.graph.triples((None, self.spdx_namespace.fileName, Literal(pkg_file.name))))
          if len(nodes) == 1:
              return nodes[0][0]
          else:
              raise InvalidDocumentError('handle_package_has_file_helper could not' +
                                         ' find file node for file: {0}'.format(pkg_file.name))
docs: Return node representing pkg_file
          pkg_file should be instance of spdx.file.