T15807.hs:12:3: error:
    • Cannot generalise type; skolem ‘f’ would escape its scope
      if I tried to quantify (f0 :: f -> *) in this type:
        forall f (a :: f). f a %1 -> App @f @f0 a
      (Indeed, I sometimes struggle even printing this correctly,
       due to its ill-scoped nature.)
    • In the definition of data constructor ‘MkApp’
      In the data declaration for ‘App’