installing pl/pgsql

Howdy:

I'm running Postgres 7.1.3 on Mandrake Linux 8.0, kernel
version 2.4.16.

I'm reading through a bit of documentation regarding pl/pgsql
and I don't see too much about it. As for installing it, the
documentation says, 'by default, it's there', but how can I
tell?

I thought I'd try this as an example:

[snip]

CREATE FUNCTION a_function () RETURNS int4 AS '
DECLARE
an_integer int4;
BEGIN
an_integer := 10 * 10;
return an_integer;
END;
' LANGUAGE 'plpgsql';

[/snip]

But I get an error saying something about no such language,
use C, sql, etc ...

Now, having said all of that ...

I would like to install it, but I'm not sure what the ramifications
are. (Should I stop and restart Postgres? How does this effect
the existing database / table? Should I install anything else
before / after this? Is it as straightforward as
http://www.postgressql.info/x5371.htm
makes it out to be?)

Any suggestions or pointers would be appreciated

TIA

-X