# File format.rb, line 730
    def setBottom (format)
      bottom_format = format
      if (!format.is_a?(Format))
        bottom_format = Format.new(format);
      end
      raise FormatException, "recursive format not allowed" if (bottom_format == self)
      @bottom = bottom_format
      #in case we've already set use_hash
      useHash( @use_hash )

    end