[exim] Condition rule, matching Subject header

Hi,

I'd like to act on messages which contain a particular string in their subject. I've
created the following rule:

  warn message = Subject: something else
        condition = ${if eq {$h_Subject:}{something}{yes}{no}}

To my knowledge, any message with "something" as the subject should have the subject
rewritten to "something else".

Ensim seems happy with it's syntax (no warnings either at start up or during mail
processing), but the subject is never changed, so I'm assuming the condition never
matches.

Thanks,
Peter Smith