def backward_char

      file   = @file
      result = nil

      if file.pos > 0
        file.seek(-1, IO::SEEK_CUR)
        result = file.getc
      end

      result
    end
def backward_char

      file   = @file
      result = nil

      if file.pos > 0
        file.seek(-1, IO::SEEK_CUR)
        result = file.getc
      end

      result
    end
get a backword character from file#pos

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