cleaning up assignments for summer 2007 project management

the limitation of one constraint per var doesn't allow, e.g. "after the 1st and
before the 15th"

They're oh-so-bad. They need to be rearchitectured to be much more general, i.e.
take in the entire set of bindings, instead of directly acting on one binding.
This would also fix the problem with the hash from variables to filters, which
limits the filters on a variable to 1.

What we need is just an array of filters that the set of bindngs is checked
against every time a new variable is bound. This will slow down the query, but
what can you do.