of play is the best that each player can do (see Figure 1). This is called the <em>principal variation</em>. Generally, the deeper an engine can search this tree without missing anything important, the stronger it will be. Most of the progress in classical engines during the previous few decades has come from pruning this tree; that is, working out which moves and possibilities do not need to be calculated because they are not good enough, too good, not interesting enough, and so on, and skipping them. This