# File format.rb, line 712
    def setTop (format)
      top_format = format
      if (!format.is_a?(Format))
        top_format = Format.new(format)
      end
      raise FormatException.new(), "recursive format not allowed" if (top_format == self)
      @top = top_format 
      #in case we've already set use_hash
      useHash( @use_hash )

    end