absolute value fro timestamps

Hello

Evidently is possible to have a negative time interval:

clapidus=> select interval '-1';
 interval
-----------
 -01:00:00

However, there seems to be no provision to get the absolute value in such
case:

clapidus=> select @ interval '-1';
ERROR:  operator does not exist: @ interval
HINT:  No operator matches the given name and argument type(s). You may need
to add explicit typecasts.

clapidus=> select abs(interval '-1');
ERROR:  function abs(interval) does not exist
HINT:  No function matches the given name and argument types. You may need
to add explicit typecasts.

What did I miss this time?

PGversion is 7.4b2

cl.