docs: def add(self, name, attributes):
          """
          Add the relation to the Schema.
          :param name: The name of a relation.
          :param attributes: A list of attributes for the relation.
          :raise RelationReferenceError: Raised if the name already exists.
          """
          if name in self._data:
              raise RelationReferenceError(
                  'Relation \'{name}\' already exists.'.format(name=name))
          self._data[name] = attributes[:]