code: function (args, env) {
          var lastSelector = this.selectors[this.selectors.length-1];
          if (!lastSelector.evaldCondition) {
              return false;
          }
          if (lastSelector.condition &&
              !lastSelector.condition.eval(
                  new(tree.evalEnv)(env,
                      env.frames))) {
              return false;
          }
          return true;
      }
docs: lets you call a css selector with a guard