T7294.hs:25:5: Warning:
    Couldn't match type ‛'False’ with ‛'True’
    Inaccessible code in
      a pattern with constructor
        Nil :: forall a. Vec a 'Zero,
      in an equation for ‛nth’
    In the pattern: Nil
    In an equation for ‛nth’: nth Nil _ = undefined