# File htree/output.rb, line 168
    def generate_content
      result = ''
      if @public_identifier
        result << "PUBLIC \"#{@public_identifier}\""
      else
        result << "SYSTEM"
      end
      # Although a system identifier is not omissible in XML,
      # we cannot output it if it is not given.
      if @system_identifier
        if /"/ !~ @system_identifier
          result << " \"#{@system_identifier}\""
        else
          result << " '#{@system_identifier}'"
        end
      end
      result
    end