>>11126181>1. Free pieces 2. Trapped pieces.The most obvious free stone in the OP picture is at a4. The other obvious free stones are yet to be placed.
f4 makes things a little complicated. It can be defined as a free stone because it will have to end up moving at some point.
d1 is also the best spot for a free piece, as mentioned earlier. In fact, it might even be worth considering putting 9. d1.
I just noticed another possible mill. Namely, column a. This possible threat could also force f4 to move. Of course, a mill on row 1, alluded to earlier, can be a threat as well.
Any mill that allows a one-two punch is desirable. The second punch being on row 5. Which is why a mill on the f column is only a last resort. For the same reason, the stone on f4 can be considered free.
Anyway, the goal is to reduce the size of the problem. It's worth noting that there are only seven nodes where the four free stones can go.
With strong play, all games should result in a position with similarly limited choices.
Following first principles greatly reduces the number of calculations. c5 wasn't really necessary. Suppose g4 or g1 had been played instead. Then e5 wouldn't be there. Yet, column c and row 3 would still be trapped. It might not result in a better position, but it's worth calculating.
The point is to trap pieces. There aren't very many ways to trap pieces. Therefore, the concept of trapping pieces greatly reduces the size of the problem.
