+Note [Only functions!]
+~~~~~~~~~~~~~~~~~~~~~~
+Consider the following code
+
+ f = g (case v of V a b -> a : t f)
+
+where g is expensive. If we aren't careful, liberate case will turn this into
+
+ f = g (case v of
+ V a b -> a : t (letrec f = g (case v of V a b -> a : f t)
+ in f)
+ )
+
+Yikes! We evaluate g twice. This leads to a O(2^n) explosion
+if g calls back to the same code recursively.
+
+Solution: make sure that we only do the liberate-case thing on *functions*
+