Consider the following code

     f = g (case v of
              V a b -> a : t f)

where g is expensive. 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.

This happen sometimes in HPC, because every tick is a liberate-able case,
but is a general problem to case liberation (I think).