T3330a.hs:17:28:
    Couldn't match type `ix' with `ix2'
      because this skolem type variable would escape: `ix2'
    This skolem is bound by
      the polymorphic type `forall ix. phi ix -> r ix -> m (r' ix)'
    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)

T3330a.hs:17:34:
    Couldn't match type `s' with `(->) (s1 ix1 -> ix)'
      `s' is a rigid type variable bound by
          the type signature for `children' at T3330a.hs:16:13
    Expected type: (s1 ix1 -> ix) -> r ix -> Writer [AnyF s] (r' ix)
      Actual type: s ix2
    In the first argument of `hmapM', namely `p'
    In the first argument of `execWriter', namely `(hmapM p collect x)'

T3330a.hs:17:36:
    No instance for (HFunctor (PF s))
      arising from a use of `collect'
    Possible fix: add an instance declaration for (HFunctor (PF s))
    In the second argument of `hmapM', namely `collect'
    In the first argument of `execWriter', namely `(hmapM p collect x)'
    In the expression: execWriter (hmapM p collect x)

T3330a.hs:17:44:
    Couldn't match type `ix1' with `r ix -> Writer [AnyF s1] (r ix)'
      `ix1' is a rigid type variable bound by
            the type signature for `children' at T3330a.hs:16:15
    In the third argument of `hmapM', namely `x'
    In the first argument of `execWriter', namely `(hmapM p collect x)'
    In the expression: execWriter (hmapM p collect x)