MMD backwards compatibility

[Dan Sugalski]

Since we really ought to have it, as I don't think we want to bother 
messing around with what we have already in the PMCs, and it's useful 
in those cases where folks want to write a single override version 
and install it as a default...

What I'm planning on doing is setting it so that a type of 0 for the 
left or right side is the "default", what's called if we can't find a 
real thing to call. The search order will be:

*) A match indexed by left and right types (including an inherited match)
*) A match where the right type is 0
*) A match where the left type is 0
*) The table default
This is a virtual search list--we can (and probably ought) to have 
the table pre-populated so that an indexed lookup is guaranteed to 
succeed. On the other hand, doing that with a zillion types will lead 
to really big tables, so we may find it cheaper in some cases to 
actually do the search, rather than prepopulate the tables.
--
                                        Dan

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Dan Sugalski                          even samurai
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