Anonymous: I have a (newbie) doubt on how to count territories, hope you can help me. The below image shows the territory counting performed by the GnuGo software. I don't think I got the procedures for assigning territories perfectly, so my question is: Why does the left area belong to neither Black nor White? Shouldn't that area be White's? (since after all he has a chain from the top to bottom of the board surrounding it)
MrTenuki: To my SDK eyes, the game is not over because the left side appears unsettled. For example, White could add a move to keep Black's five stones dead, while Black might be able to make life or seki by adding a move.
symplicity: The condition for owning an area is NOT just surrounding it. Rather, you own an area if you are able to kill every stone the opponent plays in it.
The reason you often see the opponent's stones left over in an area in games between experienced players is only because both players have agreed that those stones are dead no matter what, and that it would be a waste of time for both players to tediously go through the motions of actually capturing them. But this is ONLY by mutual agreement - if the players disagree, then they should keep playing until the stones are actually captured, or actually survive, or until it otherwise becomes obvious. (Important: as a beginner, you should use "Area scoring" or "Chinese scoring" rules, which allow you to play out a position without affecting the final score. If you use "Territory scoring" or "Japanese scoring", playing extra "unnecessary" moves to clarify a confusing position might actually affect the score).
So in this case, if you are white, and you think that area belongs to you and GnuGo disagrees, then you should keep playing until GnuGo agrees that his black stones will die and that you own it, or until Gnugo proves you wrong by making them survive. And vice versa if you are black and Gnugo is white.