def group_by_power(&block)
      power = 0
      @number.split('').reverse.each_slice(3) do |digit|
        yield power, digit
        power += 3
      end
    end
def group_by_power(&block)
      power = 0
      @number.split('').reverse.each_slice(3) do |digit|
        yield power, digit
        power += 3
      end
    end
'1234567' #=> {0=>["7", "6", "5"], 3=>["4", "3", "2"], 6=>["1"]}