Hard to answer. I think the main thing is that there are a lot of stumbling blocks, things that most people overlook, yet logical once you have it explained. O

Hard to answer. I think the main thing is that there are a lot of stumbling blocks, things that most people overlook, yet logical once you have it explained. Occasionally one is discovered and solved, and it can lead to a brief domino effect, but I have no idea how many are left.