concatenating hstores in a group by?

Hi,

I have a table with a timestamp and hstore columns with times in milliseconds.

I want to concatenate the hstores, grouped by timestamp, with the timestamps truncated to whole seconds: date_trunc('seconds', timer).

How can I concatenate all the hstores within a one second interval,

 ie, something like:

select timer,

              <hstores concatenated>

from (select  date_trunc('seconds', timer) as timer_sec, hstore)

group by timer;

Thanks