+Note [Eta-expanding INLINE things]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ foo :: Eq a => a -> a
+ {-# INLINE foo #-}
+ foo x = ...
+
+If (foo d) ever gets floated out as a common sub-expression (which can
+happen as a result of method sharing), there's a danger that we never
+get to do the inlining, which is a Terribly Bad thing given that the
+user said "inline"!
+
+To avoid this we pre-emptively eta-expand the definition, so that foo
+has the arity with which it is declared in the source code. In this
+example it has arity 2 (one for the Eq and one for x). Doing this
+should mean that (foo d) is a PAP and we don't share it.
+
+Note [Nested arities]
+~~~~~~~~~~~~~~~~~~~~~