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In the new checkers variants, captured stones remain on the board until you've hit "move". This is according to the rules, but it still startles me each time it happens to me (probably because it doesn't happen in regular Checkers).
Would it be possible to mark captured, but not removed checkers? For instance, with a red (or some other distinctive colour) around the stone, just as we have blue squares indicating the start and end square of the moving piece.
Fencer: That would also help to visually understand why you can't capture a specific piece because usually you can't capture the same piece twice. If it's highlighted somehow then you immediately know why.
pauloaguia: You immediately know why because BrainKing does not allow you to do so and good users don't think about BrainKing's system decisions. You are right, of course.