tcfail201.hs:18:58:
    Couldn't match type `a' with `HsDoc t0'
      `a' is a rigid type variable bound by
          the type signature for
            gfoldl' :: (forall a1 b. c (a1 -> b) -> a1 -> c b)
                       -> (forall g. g -> c g)
                       -> a
                       -> c a
          at tcfail201.hs:16:1
    Expected type: HsDoc t0 -> a
      Actual type: HsDoc t0 -> HsDoc t0
    In the first argument of `z', namely `DocParagraph'
    In the first argument of `k', namely `z DocParagraph'