def ack(m, n)
  if (m == 0)
    n + 1
  elsif (n == 0)
    ack(m - 1, 1)
  else
    ack(m - 1, ack(m, n - 1))
  end
end

Bench.run [7, 9] do |n|
  ack(3, n)
end