def __init__(self, path, lineno, lexpos, name, extends, doc, fields,<EOL>examples, closed=False):
super(AstUnionDef, self).__init__(<EOL>path, lineno, lexpos, name, extends, doc, fields, examples)<EOL>self.closed = closed<EOL>
Args:
    closed (bool): Set if this is a closed union.

See AstTypeDef for other constructor args.