Index on array element

How can I create an index on an array element? I seem to recall having
done this in the past but I don't recall how.

steve=# \d foo
       Table "public.foo"
  Column   |  Type  | Modifiers
-----------+--------+-----------
 textarray | text[] |

steve=# create index foodex on foo (textarray[3]);
ERROR:  parser: parse error at or near "[" at character 38

steve=# select version();
PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.3
20030226 (prerelease) (SuSE Linux)

Cheers,
Steve