Сreate parallel aggregate

Hello, everyone!

I was trying to create a parallel aggregate with base_type parameter and failed

postgres=# CREATE AGGREGATE ST_Extent_parallel (         
        sfunc = ST_CombineBBox,
        combinefunc = ST_CombineBBox,
        finalfunc = box2d,
        stype = box3d,
        basetype = geometry,
        parallel = safe
        );
ERROR:  syntax error at or near "parallel"
LINE 7:         parallel = safe

But everything is ok if I use arg_data_type:

postgres=#  CREATE AGGREGATE ST_Extent_parallel(geometry) (
        sfunc = ST_CombineBBox,
          combinefunc = ST_CombineBBox,
        finalfunc = box2d,
        stype = box3d,
        parallel = safe
        );
CREATE AGGREGATE

Is that a bug or a feature?

-- 
Grigory Smolkin
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company