This is Stockfish 2.1. Diff stats from 2.0.1 says: 54 files changed, 4138 insertions(+), 5656 deletions(-) So more than 1500 lines of code removed for this release. In this release a new skill level facility has been introduced. Stockfish can be set to skill level 20 (default) to play at maximum strength or, through setting the “Skill Level” UCI option, can be lowered until 0 when should be beatable even by weak players.
