Pg/PLSQL Errors!!

I am running PostgreSQL 6.5.3 on RedHat Linux 6.1 on a PC.
I am trying to use Pg/PLSQL.

I even tried being conservative enough to copy the example code from
http://www.postgresql.org/docs/user/c40874340.htm as follows:
CREATE FUNCTION add_one (int4) RETURNS int4 AS '
BEGIN
RETURN $1 + 1;
END;
' LANGUAGE 'plpgsql';

though, even that results in "ERROR: Unrecognized language specified in a CREATE FUNCTION: 'pl-pgsql'. Recognized languages are sql, C, internal and
the created procedural languages."

I have tried variants including PLSQL,PG/PLSQL,PL/SQL,PGSQL and even 'internal
procedural language(s)' and 'created procedural language(s)' though with the same error.

Does anyone know of the name of the postgreSQL's procedural language of form like
CREATE function funcName(arguments) returns returntype AS 'BEGIN
statement block END;' LANGUAGE '??????';

Many Thanks!

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