tcfail131.hs:7:9:
    Could not deduce (b ~ Integer)
    from the context (Num b)
      bound by the type signature for g :: Num b => b -> b
      at tcfail131.hs:6:8-22
      `b' is a rigid type variable bound by
          the type signature for g :: Num b => b -> b at tcfail131.hs:6:8
    Relevant bindings include
      g :: b -> b (bound at tcfail131.hs:7:3)
      x :: b (bound at tcfail131.hs:7:5)
    In the return type of a call of `f'
    In the expression: f x x
    In an equation for `g': g x = f x x