Index on substring?

I thought this was possible, but searching the archives & docs I can't find
any reference to it...
Am I doing something wrong?

jeffe(at)kiyoko=> psql -V
psql (PostgreSQL) 7.0.0

jeffe(at)kiyoko=> uname -a
FreeBSD kiyoko.la.verio.net 4.0-STABLE FreeBSD 4.0-STABLE #0: Thu Apr 27
10:44:07 CDT 2000 root@:/usr/src/sys/compile/KIYOKO i386

extracts=# create index c_namesum_i on customers (substr
(bill_company,1,5));
ERROR: parser: parse error at or near "1"
extracts=# select substr (bill_company, 1, 5) from customers limit 10;
substr
--------
RoadW
Beliz
Radio
Trill
R2000
Data
Inter
AEC M
G2 Gr
MindB
(10 rows)

extracts=# create index c_namesum_i on customers (substring(bill_company
from 1 for 5));
ERROR: parser: parse error at or near "substring"
extracts=# select substring (bill_company from 1 for 5) from customers limit
10;
substr
--------
RoadW
Beliz
Radio
Trill
R2000
Data
Inter
AEC M
G2 Gr
MindB
(10 rows)

extracts=#