def weirdFold[T,B](s:List[T], init:B, f:(T,List[T], B) => (T,B)) = {
    val v = s.toVector
    val (nv, nb, _) = v.foldLeft((v, init, 0)) { case ((ts, b, i), t) =>
      val (newT, newB) = f(t, s, b)
      (ts.updated(i, newT), newB, i+1)
    }
    (nv.toList, nb)
  }