If you want a "The list is cleared! Hurray!" - effect in your GTD system, the NA lists are the wrong place to look at. NAs are about beginning work, not finishing. You can creating lists to clear by grouping your projects. For instance you can have a group of projects that you want to complete 'next'. Once every project on that list is done, you arrived at 'next'. You cleared the list, know you can shout your Hurray!