Trigger problem

Hi,
I am new to postgresql and to this moment I am using oracle.
I use posgresql 7.1.2

I try to create very simple ( from doc. examples ) trigger:

CREATE FUNCTION add_one (integer) RETURNS INTEGER AS '
    BEGIN
        RETURN $1 + 1;
    END;
' LANGUAGE 'pgsql';

But the system report:
ERROR:  Unrecognized language specified in a CREATE FUNCTION: 'pgsql'.
    Recognized languages are sql, C, internal, and created procedural
languages.

I readet in documentation that pgsql is good solution for SP.

Where is the problem?

Many thanks in advance.

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