Select from function for part of column results

I've written a function to provide information relating to each row in
an inbox. I'd like to do a query returning some information from each
row of the inbox + some columns from the function pertinent to each row
returned.

I've tried the following on 7.4.6:

    t4=> select * from function_message_context(inbox.rowid), inbox;
    ERROR:  function expression in FROM may not refer to other
            relations of same query level

Is there a way I can get the function to provide some columns in the
query?

Thanks
Rory

--
Rory Campbell-Lange
<rory@campbell-lange.net>
<www.campbell-lange.net>