tcfail099.hs:9:20:
    Couldn't match expected type `a' with actual type `t'
      because type variable `a' would escape its scope
    This (rigid, skolem) type variable is bound by
      a pattern with constructor
        C :: forall a. (a -> Int) -> DS,
      in an equation for `call'
      at tcfail099.hs:9:7-9
    Relevant bindings include
      call :: DS -> t -> Int (bound at tcfail099.hs:9:1)
      f :: a -> Int (bound at tcfail099.hs:9:9)
      arg :: t (bound at tcfail099.hs:9:12)
    In the first argument of `f', namely `arg'
    In the expression: f arg
    In an equation for `call': call (C f) arg = f arg