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) from ledger 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