let n = 4
let buf = Array.Empty(n)

func rec(idx, begin) {
    for i in begin..<n {
        buf[idx] = i
        for j in 0..idx {
            print("{0, 2}".Format(buf[j]), terminator: "")
        }
        print("")
        rec(idx + 1, buf[idx] + 1)
    }
}

rec(0, 0)