tcfail131.hs:7:13:
    Could not deduce (b ~ Integer)
    from the context (Num b)
      bound by the type signature for g :: Num b => b -> b
      at tcfail131.hs:7:3-13
      `b' is a rigid type variable bound by
          the type signature for g :: Num b => b -> b at tcfail131.hs:7:3
    In the second argument of `f', namely `x'
    In the expression: f x x
    In an equation for `g': g x = f x x