You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: trackback.rb
    path of file: ./repos/truffleruby/lib/gems/gems/rss-0.2.9/lib/rss
    
    the code of the file until where you have to start completion: # frozen_string_literal: false

    
    def append_features(klass)
      super

      unless klass.class == Module
        klass.module_eval {include TrackBackUtils}

        klass.install_must_call_validator(TRACKBACK_PREFIX, TRACKBACK_URI)
        %w(ping).each do |name|
          var_name = "#{TRACKBACK_PREFIX}_#{name}"
          klass_name = "TrackBack#{Utils.to_class_name(name)}"
          klass.install_have_child_element(name, TRACKBACK_URI, "?", var_name)
          klass.module_eval(<<-EOC, __FILE__, __LINE__)
            remove_method :#{var_name}
            def #{var_name}
              @#{var_name} and @#{var_name}.value
            end

            remove_method :#{var_name}=
            def #{var_name}=(value)
              @#{var_name} = Utils.new_with_value_if_need(#{klass_name}, value)
            end
          EOC
        end

        [%w(about s)].each do |name, postfix|
          var_name = "#{TRACKBACK_PREFIX}_#{name}"
          klass_name = "TrackBack#{Utils.to_class_name(name)}"
          klass.install_have_children_element(name, TRACKBACK_URI, "*",
                                              var_name)
          klass.module_eval(<<-EOC, __FILE__, __LINE__)
            remove_method :#{var_name}
            def #{var_name}(*args)
              if args.empty?
                @#{var_name}.first and @#{var_name}.first.value
              else
                ret = @#{var_name}.__send__("[]", *args)
                if ret.is_a?(Array)
                  ret.collect {|x| x.value}
                else
                  ret.value
                end
              end
            end