box function implemented?

I think I found a glitch either in the box function or in the docs for
postgresql 7.3.2. The online docs say that the box function can find the
intersection box between two boxes. I created a table that looks like this:
CREATE TABLE boxtest (id serial primary key, b1 box, b2 box);

and inserted a row with boxes for b1 and b2. All seemed great. But when I
try:
SELECT box(b1, b2) from boxtest;

I get this error:
Function box(box, box) does not exist

Followed by a couple more lines suggesting I use typecasts. But the docs at
http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=functions-geometry.html
 give exactly that prototype for the box function.

Having said all that, I have to say that postgresql's geometry functions
are fantastic. I just came up with my first need for them, and it looks
like they're going to really save me from writing a chunk of code and speed
things up. Thanks!

Wes