T14765.hs:11:31: error:
    • The function argument
        ‘(k proxy#)’
      does not have a fixed runtime representation:
        r :: TYPE rep
    • In the first argument of ‘f’, namely ‘(k proxy#)’
      In the second argument of ‘fold’, namely ‘(f (k proxy#) x)’
      In the expression: fold f (f (k proxy#) x) xs