type toDo = {id: string; indent: int; descendants: toDo list}

let rec children x ys accumulated =
        match ys with
            | [] -> (accumulated, [])
            | y::rest ->
                match (y.indent - x.indent) with
                    | 1 ->
                        let descendants, _ = children y rest List.empty<toDo>
                        let filledChild = {y with descendants = descendants |> List.sortBy (fun x -> x.id)}
                        filledChild::accumulated |> children x rest
                    | diff when diff < 1 -> (accumulated |> List.sortBy(fun x -> x.id), y::rest)
                    | _ -> children x rest accumulated