def backward_char

      file    = @file
      pos_max = file.pos - 1
      pos_min = pos_max - @mb_bytesize_max

      pos_max.downto(pos_min) do |pos|

        break if pos < 0

        file.seek(pos)
        char = file.getc

        # return file#getc character
        # - when that is regular for multibyte character
        return char if check_mb(char)
      end

      nil
    end
def backward_char

      file    = @file
      pos_max = file.pos - 1
      pos_min = pos_max - @mb_bytesize_max

      pos_max.downto(pos_min) do |pos|

        break if pos < 0

        file.seek(pos)
        char = file.getc

        # return file#getc character
        # - when that is regular for multibyte character
        return char if check_mb(char)
      end

      nil
    end
get a backword character from file#pos

 instance variables
   mb_bytesize_max ... max bytesize for multibyte character

 args
   update_flag     ... if true, update file#pos for backward character's head

 returner
   String object ... exists
             nil ... not exists