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.

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[:]