Firing trigger if only

Good afternoon all.I have some problem with triggers on PostgreSQL 8.4.I have
a trigger on specific table(articles) that fires on update statement:

CREATE OR REPLACE FUNCTION trigger_articles_update()
RETURNS trigger AS
$BODY$BEGIN
INSERT INTO
article_change(article_id,change_type)VALUES(OLD.article_id,2);
RETURN NULL;
END$BODY$
LANGUAGE 'plpgsql' VOLATILE
COST 100;
ALTER FUNCTION trigger_articles_update() OWNER TO postgres;

I have 2 different applications that performs update on table
articles(written in Delphi and using ZeosDB). My problem is that I want
trigger to fire only when performing update with first application, but not
with second.I know that triggers supposed to fire on every change on table,
but this is a specific problem that I have.Any hint appreciated. 8)

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