// f : L -> L
static L xtok_recurse_limited_debug(O f, L l, int limit) {
  print("xtok_recurse " + f + " limit " + limit + " on " + struct(l));
  l = (L) callF(f, l);
  print("xtok_recurse now: " + struct(l));
  if (limit <= 1) ret l;
  for (int i = 1; i < l(l); i += 2) {
    O o = l.get(i);
    if (o instanceof L)
      l.set(i, xtok_recurse_limited_debug(f, (L) o, limit-1));
  }
  ret l;
}