🕸Fergus Duniho wrote on Mon, May 4, 2020 12:07 PM UTC:
I don't think so. The Emperor does have a pseudo-legal move to that square. That it cannot use it is just because it would expose itself (the royal) to capture, just as moving a pinned piece would.
Thank you for this example. It shows we are not talking about the same thing. What you're calling pseudo-legal is a convenient shortcut for distinguishing what is easy to compute from what takes more time to compute. What I'm talking about is a conceptual difference between a piece's own powers of capture and the universal restriction on all pieces against creating an illegal position. In the case of the Emperor, its inability to capture on a protected space may require more computation than usual, but it is an inherent part of that piece's ability to capture. So, conceptually speaking, regardless of how you choose to program it, the ability to move to an unprotected space is not included in its powers of capture. If you claim that it has a pseudo-legal ability to capture on a protected space, then this shows that pseudo-legal is not a useful concept for describing what attack means.
Also, having read the relevant parts of the Tai Shogi rules, the reason behind the restriction is to prevent the first player from taking the opponent's Prince or Emperor on the very first move. Unlike most Chess variants, this game has two royal pieces, and it is legal, though not usually advisable, to leave a Prince or Emperor exposed to capture. The Wikipedia page mentions that a player could choose to sacrifice one of his two royal pieces in a gambit. Although the Emperor may not capture on a space where it would be under attack, it is allowed to move to an empty space where it would be exposed to capture. The Prince moves like a King, but it is also allowed to move into or remain in check. So, the restriction on the Emperor's ability to capture is not because it would create an illegal position like moving a pinned piece would.
Thank you for this example. It shows we are not talking about the same thing. What you're calling pseudo-legal is a convenient shortcut for distinguishing what is easy to compute from what takes more time to compute. What I'm talking about is a conceptual difference between a piece's own powers of capture and the universal restriction on all pieces against creating an illegal position. In the case of the Emperor, its inability to capture on a protected space may require more computation than usual, but it is an inherent part of that piece's ability to capture. So, conceptually speaking, regardless of how you choose to program it, the ability to move to an unprotected space is not included in its powers of capture. If you claim that it has a pseudo-legal ability to capture on a protected space, then this shows that pseudo-legal is not a useful concept for describing what attack means.
Also, having read the relevant parts of the Tai Shogi rules, the reason behind the restriction is to prevent the first player from taking the opponent's Prince or Emperor on the very first move. Unlike most Chess variants, this game has two royal pieces, and it is legal, though not usually advisable, to leave a Prince or Emperor exposed to capture. The Wikipedia page mentions that a player could choose to sacrifice one of his two royal pieces in a gambit. Although the Emperor may not capture on a space where it would be under attack, it is allowed to move to an empty space where it would be exposed to capture. The Prince moves like a King, but it is also allowed to move into or remain in check. So, the restriction on the Emperor's ability to capture is not because it would create an illegal position like moving a pinned piece would.