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These diagrams are a wonderful thing. They can easily describe various moves that Game Courier can't handle without using complicated code. However, I have a few suggestions that, in my opinion, would make the diagram even better.

• Tame pieces - The modifier t on a final (or only) leg of a move means that move cannot capture royal pieces in XBoard. In other words, it is the opposite of the modifier k on a final (or only) leg of a move, which means that move can only be used to capture royal pieces. This modifier should be relatively easy to implement.
• Custom definition of royal pieces - Currently, the only piece that is specified as royal by the diagram is the last piece on the diagram. Some variants may have more than one kind of royal piece (i.e. Chu Shogi), which makes this a useful extension, especially if the royal pieces must stay out of check.
• Triple Captures - This is without a doubtthe hardest extension to implement. Adding this would, in theory, require adding a lot more code to handle the second locust capture. However, this extension would be necessary for the Lion Dog, which can capture three pieces in a straight line, and other pieces that can capture three pieces at once. There might be a way to do this with the WierdChain function. However, that function hasn't been used in your diagrams other than in the Paco Shako diagram.