How do I create a box from fields in a table?

I need to determine whether a given pair of coordinates is inside a given rectangle. According to the documentation, PostgreSQL provides the box and point types and a "contains" operator that will be perfect for this. However, the example provided in the documentation only shows the creation of boxes and points from constant values. So, the following query works:

select tran_car_identification, box '((0, 1), (2, 3))' from transfer_car

But if I try to use a field from the transfer_car table, it doesn't work:

select tran_car_identification, box '((location, 1), (2, 3))' from transfer_car

That gives me an "invalid input syntax for type box" error.

How do I create a box object using data from the transfer_car table?

Thanks very much!

RobR