cannot index functions with multiple arguments

Scott Marlowe (smarlowe(at)ihs(dot)com) reports a bug with a severity of 2
The lower the number the more severe it is.

Short Description
cannot index functions with multiple arguments

Long Description
Postgresql version: 7.1beta2
OS: RedHat Linux 7.0

Postgresql allows indexing of functions applied to columns, like so:

create index yada on table (lower(column));

but this fails on functions like substr that have multiple arguments.

create index yada on table (substr(column,1,4));

fails like so:

ERROR: parser: parse error at or near "1"

Sample Code
Exact code as I typed it in, with response:

smarlowe=# create index exten_extension_idx on exten (substr(extension,1,4));
ERROR: parser: parse error at or near "1"

I hope this isn't a known bug and I'm reporting it again.

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