code: def set_pkg_file_name(self, doc, name):
          
          self.assert_package_exists()
          if not self.package_file_name_set:
              self.package_file_name_set = True
              doc.package.file_name = name
              return True
          else:
              raise CardinalityError('Package::FileName')
docs: Sets the package file name, if not already set.
          name - Any string.
          Raises CardinalityError if already has a file_name.
          Raises OrderError if no pacakge previously defined.