Lee Se-dol, a former international champion of the game Go, has announced his retirement from competitive play. He became world-famous after losing a set of matches to AlphaGo, the matchplaying software developed by Google's DeepMind artificial intelligence unit. Until 2016, Go was considered so complex that software might be incapable of mastering it. There are almost an infinite number of gameplay combinations, giving humans -- with their instincts and gut feel for where the game is going -- an advantage over machines. But, "with the debut of AI in Go games, I've realized that I'm not at the top even if I become the No. 1 through frantic efforts," Lee told the Yonhap News Agency. "Even if I become the No. 1, there is an entity that cannot be defeated."