passing values into .sql scripts

I want to do something like this:
    \set tmp :acct 'a value'    \i query.sql

where query.sql looks like this:
    select sum(amount), dr_acct from ledger where dr_acct = :acct  
group by dr_acct;    select sum(amount), cr_acct from ledger where cr_acct = :acct  
group by cr_acct;    select       (select sum(amount) from ledger where dr_acct = :acct )     - (select sum(amount)
fromledger where cr_acct = :acct );
 

However, this is what I get:
    psql:pnc.sql:1: ERROR:  column "a value" does not exist

Is there a different way I should be doing this?  (I'm used to  
Oracle's &1...&n parameters.)

Geoffrey
--
Geoffrey S. Knauth | http://knauth.org/gsk