[dev] Forwards driver question

[Kevin M. Myer]

I looked at the Forwards module and saw the big TODO of merging it
with Ingo.  Since forwarding could be considered a specialized case of
filtering, Ingo then becomes a frontend to manage your general filters
and your special forward filters.  The backends for each could be the
same, similar, or totally different, and you could have both general
and forward filters, or general filters, or forward filters.  Some
backends already support forwarding, so the general filter driver
should know about this and disable the forward functionality the
Forward module would provide if its already provided by the general
backend.

Now, my question would be:
is there an underlying design principle that would guide how to best
go about this?  Should Ingo be treated as consisting of a generic
backend, which can be extended for specific functionality (vacation,
forward, spam, white/blacklist), if the backend doesn't already
support that?  Or should each additional functionality be treated
separately and independently?

Kevin