+Note [Expanding newtypes]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+When expanding a type to expose a data-type constructor, we need to be
+careful about newtypes, lest we fall into an infinite loop. Here are
+the key examples:
+
+ newtype Id x = MkId x
+ newtype Fix f = MkFix (f (Fix f))
+ newtype T = MkT (T -> T)
+
+ Type Expansion
+ --------------------------
+ T T -> T
+ Fix Maybe Maybe (Fix Maybe)
+ Id (Id Int) Int
+ Fix Id NO NO NO
+
+Notice that we can expand T, even though it's recursive.
+And we can expand Id (Id Int), even though the Id shows up
+twice at the outer level.
+
+So, when expanding, we keep track of when we've seen a recursive
+newtype at outermost level; and bale out if we see it again.
+
+