code: def set_pkg_down_location(self, doc, location):
          
          self.assert_package_exists()
          if not self.package_down_location_set:
              self.package_down_location_set = True
              doc.package.download_location = location
              return True
          else:
              raise CardinalityError('Package::DownloadLocation')
docs: Sets the package download location, if not already set.
          location - A string
          Raises CardinalityError if already defined.
          Raises OrderError if no package previously defined.