def split_isbn_number number
      if number.is_a?(String)
        [ number[0..-2], (%w[X x].include?(number[-1]) ? "X" : number[-1].to_i) ]
      else
        number.divmod(10)
      end
    end
def split_isbn_number number
      if number.is_a?(String)
        [ number[0..-2], (%w[X x].include?(number[-1]) ? "X" : number[-1].to_i) ]
      else
        number.divmod(10)
      end
    end
[split_isbn_number description]
 @param  number [type] [description]

 @return [type] [description]