docs: def get_attributes(self, name):
          """
          Return the list of attributes associated with the specified relation.
          :param name: A name of a relation in the Schema.
          :return: A list of attributes.
          :raise RelationReferenceError: Raised if the name does not exist.
          """
          attributes = self._data.get(name, None)
          if not attributes:
              raise RelationReferenceError(
                  'Relation \'{name}\' does not exist.'.format(name=name))
          return attributes[:]