T14765.hs:11:29: error: [GHC-55287]
    • The argument ‘(k proxy#)’ of ‘f’
      does not have a fixed runtime representation.
      Its type is:
        r :: TYPE rep
    • In the second argument of ‘fold’, namely ‘(f (k proxy#) x)’
      In the expression: fold f (f (k proxy#) x) xs
      In an equation for ‘fold’:
          fold f k (x : xs) = fold f (f (k proxy#) x) xs