T15795a.hs:9:3: error:
    • Cannot generalise type; skolem ‘u’ would escape its scope
      if I tried to quantify (cat10 :: u) in this type:
        forall u (a :: u). F @u @cat10 a
      (Indeed, I sometimes struggle even printing this correctly,
       due to its ill-scoped nature.)
    • In the definition of data constructor ‘Prod’
      In the data declaration for ‘F’