A minor question

Why do we predefine a "reply" relationship but not a "fault" 
relationship?  The idea would be that a fault would be tagged with 
(fault, requestMID) as opposed to (reply, requestMID).

I had expected to see this as a resolved issue in the issues list, but 
apparently not.  I haven't trawled through the mailing list archives, so 
apologies if it's already been done to death.