Tigger

I've look throught the docs and from what I can see the bellow code should 
work, however I keep getting the error:

ERROR:  parser: parse error at or near "$" at character 53

CREATE FUNCTION session_update() RETURNS trigger AS $session_update$
BEGIN-- Check date existsIF NEW.stamp_lastupdate IS NULL THEN    NEW.stamp_lastupdate := 'now';END IF;RETURN NEW;
END;

$session_update$ LANGUAGE plpgsql;

CREATE TRIGGER session_update BEFORE INSERT OR UPDATE ON sessions FOR EACH 
ROW EXECUTE PROCEDURE session_update();


Any help or suggestions of websites I should read would be appercated.

David

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