Opions?

[ZiNgA BuRgA]

I've been thinking about implementing a default forumdisplay filter option, except I'm not sure how I should do it.
For example, should the filter be always enforced?  Or if override-able, when should it be overridden or bypassed?
I'm thinking of implementing it as override-able, which is overridden when the user uses a different value to that specified.

For example, if I set default filter to:
filtertf_tag=Cheap
filterxt_prefix=3

And someone visits forumdisplay.php?fid=10&filterxt_icon=2, there'll be three filters active.  But if they visit forumdisplay.php?fid=10&filterxt_prefix=1 there'll be two filters active.
Array filters will override the defaults too.

Also considering whether variables, and possibly conditionals should be allowable there.

What are your opinions/suggestions?

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Also, the filter by prefix/icon/uid/lastposteruid feature added in the v1.337 is a per-forum setting, but one wonders whether this should be done globally or on a per forum basis.  I'm thinking it might make more sense as a global setting than a per-forum one.  What do you think?