be certain that the instances of the two modules have already been checked for
consistency during the compilation of modules that we import.
+Why do we need to check? Consider
+ module X1 where module X2 where
+ data T1 data T2
+ type instance F T1 b = Int type instance F a T2 = Char
+ f1 :: F T1 a -> Int f2 :: Char -> F a T2
+ f1 x = x f2 x = x
+
+Now if we import both X1 and X2 we could make (f2 . f1) :: Int -> Char.
+Notice that neither instance is an orphan.
+
How do we know which pairs of modules have already been checked? Any pair of
modules where both modules occur in the `HscTypes.dep_finsts' set (of the
`HscTypes.Dependencies') of one of our directly imported modules must have