optimizer not eliminating redundant sorts

Another odd thing. This type of query seems to be causing redundant sorts.
From what I've read in the source it tries quite hard to keep track of the
sort order of subplans to avoid unnecessary sorts, so I think these are
supposed to get optimized out. I'm not sure if the group nodes can get
optimized out, but surely the sorts can.

slo=> explain select x from (select x,y from (select x,y,z from ttt group by x,y,z) as x group by x,y) as x group by x;
                                             QUERY PLAN
----------------------------------------------------------------------------------------------------
 Group  (cost=0.05..0.06 rows=1 width=16)
   ->  Sort  (cost=0.05..0.06 rows=1 width=16)
         Sort Key: x
         ->  Subquery Scan x  (cost=0.03..0.04 rows=1 width=16)
               ->  Group  (cost=0.03..0.04 rows=1 width=16)
                     ->  Sort  (cost=0.03..0.04 rows=1 width=16)
                           Sort Key: x, y
                           ->  Subquery Scan x  (cost=0.01..0.02 rows=1 width=16)
                                 ->  Group  (cost=0.01..0.02 rows=1 width=16)
                                       ->  Sort  (cost=0.01..0.02 rows=1 width=16)
                                             Sort Key: x, y, z
                                             ->  Seq Scan on ttt  (cost=0.00..0.00 rows=1 width=16)


--
greg