For the most part, these boil down to "some global property holds, and
so this relation contains all instances of class `X`". The fix is to
explicitly build the cartesian product (which we were already building
implicitly anyway) by adding `and exists(var)` to the disjunct that did
not mention `var`.
Note that these cartesian products are always with singletons on one
side, and so should be unproblematic.
