Code. Recurrence for sequence of binary tree indices in a Fenwick array [blog/fenwick/interleaving]

pub fn[T] interleave(sel : List[T], other : List[T]) -> List[T] {
  match (sel, other) {
    (Nil, _) => Nil
    (Cons(x, xs), ys) => Cons(x, interleave(ys, xs))
  }
}

fn b(i : Int) -> List[Int] {
  match i {
    0 => Cons(2, Nil)
    n =>
      Int::until(1 << n, (1 << n) + (1 << (n - 1))).map(fn { x => x * 2 })
      |> @immut/list.from_iter
      |> interleave(b(n - 1))
  }
}