+Note [Binding when looking up instances]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When looking up in the instance environment, or family-instance environment,
+we are careful about multiple matches, as described above in
+Note [Overlapping instances]
+
+The key_tys can contain skolem constants, and we can guarantee that those
+are never going to be instantiated to anything, so we should not involve
+them in the unification test. Example:
+ class Foo a where { op :: a -> Int }
+ instance Foo a => Foo [a] -- NB overlap
+ instance Foo [Int] -- NB overlap
+ data T = forall a. Foo a => MkT a
+ f :: T -> Int
+ f (MkT x) = op [x,x]
+The op [x,x] means we need (Foo [a]). Without the filterVarSet we'd
+complain, saying that the choice of instance depended on the instantiation
+of 'a'; but of course it isn't *going* to be instantiated.
+
+We do this only for isOverlappableTyVar skolems. For example we reject
+ g :: forall a => [a] -> Int
+ g x = op x
+on the grounds that the correct instance depends on the instantiation of 'a'