    # File lib/nokogiri/css/parser_extras.rb, line 70
          def xpath_for string, options={}
            key = "#{string}#{options[:ns]}#{options[:prefix]}"
            v = self.class[key]
            return v if v

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