/// todo: replace mutable by array.reduce
let rec tally (values :obj array) =
    let mutable r = 0,0
    let add = fun s c -> r <- s + fst r, c + snd r
    values |> Array.iter(function
        | :? int as i -> add i 1
        | :? array<obj> as a when a.Length > 0 ->
            tally(a) ||> add
        | _ -> printfn "f# can't leak by accident!")
    r