# File lib/nokogiri/css/parser.rb, line 54
      def xpath_for string, options={}
        v = self.class[string]
        return v if v

        args = [
          options[:prefix] || '//',
          options[:visitor] || XPathVisitor.new
        ]
        self.class[string] = parse(string).map { |ast|
          ast.to_xpath(*args)
        }
      end