T3330a.hs:17:34:
    Couldn't match type `s' with `(->) (s ix1 -> ix1)'
      `s' is a rigid type variable bound by
          the type signature for children :: s ix -> PF s r ix -> [AnyF s]
          at T3330a.hs:17:1
    In the first argument of `hmapM', namely `p'
    In the first argument of `execWriter', namely `(hmapM p collect x)'
    In the expression: execWriter (hmapM p collect x)