CODE:

// guruguru.fsx --- ぐるぐるアスキーアート

/// n行m列の２次元配列を作る(ただしn m - x - 1)
    else if n % 2 = 0 then
        let smaller = guruguru (n - 1) (m - 1)
        Array2D.init n m (fun y x ->
            if x = m - 1 then n * (m - 1) + y
            else if y = 0 then (n - 1) * (m - 1) + x
            else Array2D.get smaller (y - 1) x )
    else
        let smaller = guruguru (n - 1) (m - 1)
        Array2D.init n m (fun y x ->
            if y = n - 1 then (n - 1) * (m - 1) + m - x - 1
            else if x = 0 then n * (m - 1) + n - y - 1
            else Array2D.get smaller y (x - 1) )

let center = "いろは歌"
let rotating = "いろはにほへとちりぬるをわかよたれそつねならむ"
               + "うゐのおくやまけふこえて"
               + "あさきゆめみしゑひもせすん"
let mask_rows = 3

let makeAsciiArt n m =
    let start = mask_rows * (mask_rows - n + m)
    let len = String.length rotating
    guruguru n m
    |> Array2D.map (fun i ->
        if i  List.init m (fun x -> ary.[y, x]))

makeAsciiArt 25 28
|> ListFromArray2D
|> List.map (fun v -> new System.String(List.toArray v))
|> List.iter (printfn "%s")