[Exim] Exim 3.x filter - rewriting vars

Hi all,

I've and system filter and I want to add a subject prefix
like [test] on subject of messages that contains
test@??? as to. This is my thinking filter contents:

if $header_to is "test@foo"
then
$header_subject = "[test] $header_subject"
endif

The line that "change" my subject is a invalid comand, of
course. How can I do this ? Is there another way out of a
system filter ?

Thanks,
Fabio