[Exim] Two questions about transport_filter, shadow_transpor…

Please forgive me if this is too elementary, but I'm really struggling
with the exim.com syntax:

1) How do I make a shadow_condition that tests for the presence of a
string in a specific message header?

2) If a message passes through a transport filter, and then I add a
shadow_transport and shadow_condition, will the check for the
shadow_condition be based on the message text before or after it is
altered by the transport filter?

Thanks in advance,
Juan