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First of all, thanks for all your input! There were many things I would never have thought of. Now some conclusions (mainly thinking aloud): It would probably be good to treat 'gating', where introduction of a piece takes places in combination with moving another piece, distinct from true drops, which leave the remainder of the board unchanged. For gating there is the choice to consider it as a side effect of the move of the piece that was already on the board, and this seems the most convenient way to do it. (In my Betza 2.0 proposal I had already introduced a move modality 'u', which could unload something on the square indicated by the Betza 'atom' to which it was prefixed.) The true drops could be considered a special kind of Betza atom, and '@' would be a good notation for it, as in standard algebraic notation drop moves are written with it (e.g. P@c6). Dropping on empty squares only could be written as m@, with the usual Betza modifier m. The power of Betza notation is in clever choice of defaults, however. Where board moves almost always have modality mc, which is thus default so it never has to be written, it is very unusual to allow capture through drops. So for the @ atom, m could be defined as the default modality. Then there is the concept of the 'drop zone'. In cases where this zone consists of a few squares scattered over the board, there seems no alternative to actually listing them. In Betza 2.0 I introduced the idea of a 'limiter', which could be put on a modality modifier, like c{N,B}A for a move that could only capture Knights and Bishops with an Alfil move. This is a bit of an awkward notation, however, so for the more common cases, where the drop zone is the entire board with the exception of some ranks at the edges, I would like to have something simpler. One method would be to write the range of ranks where the piece can be dropped, e.g. 1-7 for Shogi Knights. This conflicts with the use of the dash as chaining operator in Betza 2.0, however. It would be nice if we had to write only a single rank number, for then we could write it behind the atom: @7. This has similar syntax to W4 for a range-4 Rook. In fact it would allow a similar interpretation, when we see the drop move as a move that starts just in front of the board (on rank 0, as it were), on an arbitrary file, and slides forward from there onto the board. The number behind @ would then indicate the maximum rank it could reach. Like in the Bex proposal for extension of Betza notation, a leading 0 on the number could indicate it does not specify a maximum, but an exact number: @08 would mean 'drop on 8th rank only'. Problem with this is that it is difficult to exempt a low rank from dropping, as is needed for Crazyhouse Pawns, which can only be dropped on ranks 2-7. This would then need an exhaustive listing @02@03@04@05@06@07, which is pretty ugly. The problem is similar to that of 'skip-sliders' like the Heavenly Tetrarchs of Tenjiku Shogi, which move as a slider after skipping the first square (e.g. t(D,R) = D, then R in original Betza notation). Another problem is that the dropped piece should be able to reach any empty square within the range, even if there is blocking. IIRC original Betza notation described a piece that could jump an arbitrary number of obstacles with a 'jj' prefix. So mjj@7 would do it for Shogi Knight drops. Like the 'm', the 'jj' could be made default in absence of an explicit single 'j' or 'n'. This offers the interesting possibility to specify 'non-jumping drops' n@8, which could drop anywhere upto the first obstacle, but not beyond it. Such a drop mode was not mentioned here, but I could imagine it would be a useful restriction on drops, to not make them too aggressive. A drop on ranks 2-7 could be written with the Betza 'then' notation as t(@02,jjR5), i.e. initial drop on rank 2, followed by an arbitrary jumping forward Rook move of upto 5 squares. Still not very elegant. There seems to be no obvious way to specify further restrictions, such as on the number of Pawns per file, the number of color-bound pieces on the same color, etc., in the Betza spirit. Perhaps some extra modifiers could be introduced for such restrictions, but the available letters are already almost exhausted. Because directional modifiers make no sense on a drop, they could be reused for other purposes there. (It is already the case that the meaning of directional modifiers depends on the atom that follows; 'fr' in frW means something different than in frF or frN.) So 'f' could be taken to mean the drop is not allowed if there is already a piece of that kind on the file, and 'b' that it is not allowed if it is already on the board. 'ff' could be taken to mean there can be upto 2 on the file, etc. 's' could similarly stand for 'same color square'. This would make Shogi Pawn drops f@8, Shogi Knight drops @7, Tori-Shogi Swallow drops ff@6. Hey, that is not so bad!
Weak Square of the Jumping King, http://www.chessvariants.org/38.dir/jumpingking.html, is drop Chess. The piece(s) in hand goes only to square the King is just vacating.
Wasn't there any drop-game, where was restriction on having more than certain number pieces? Or crtain number for each kind of pieces? And also, players may have several different "places" of "pieces-in-hand" - as in Hpstage chess there are own and opposing pieces, and, of course, only friendly ones can be dropped.
One strand of restriction occurs in my Mitregi, in which a player can have no more pieces of a bound type on each binding than were on that binding at the start of the game. For example each binding strats with two Bishop, one per player, and so neither player can ever have more than two Bishops on one binding. When I first posted tha variant I had the harsher restriction of only as many as that player started with on thzt binding, but on seeing the community response I decided to relax it to the current rule. This is mainly because the slacker rule recognises that a player could end up with all the pieces of a given type, making it reasonable to be able to deploy all on each binding that started on that binding. It is also more consistent with Shogi itself, in which each player starts with one Bishop (incidentally starting on the same binding) and a player who manages to get both Bishops is free to deploy them on one binding.
Some CVs have drops to specific cells, such as Seirawan, http://www.seirawanchess.com/, Another example of that is great Altair, http://www.chessvariants.org/dpieces.dir/altair.html. There piece goes to specific rank. Lavieri accepted my calling the mechanism "drop" though in Altair the piece is not in a pocket, but already on board in a rank three away from the destination rank. Jacks and Witches is another one of those, http://www.chessvariants.org/large.dir/contest84/jacksandwitches84.html, where the off-board Jack is dropped as a move to one of four squares. Set-up phases of placement CVs have their own rules for what are legitimate drops at the beginning: Vhttp://www.chessvariants.org/large.dir/bbchess/bbchess.html. Very specialized: http://www.chessvariants.org/xiangqivariants.dir/para-xiangqi.html, because a dropped piece can go only orthogonal to guide piece (used before). ( I think all these drops limited to area are better concept than long-time Shogi free drops with minor restrictions, at least for westerners. Shogi is #1 played at GC, but same time why many players hate and won't play Shogi variants is that the pretty free drop ruins every well-thought position. )
Here are more ... *) You cannot drop a piece to your 8th rank (all pieces in Pocket Mutation Chess) ... and what about colourbound pieces? Any restrictions for keeping the original colour binding or for not having two on the same colour? (I am not aware of games stating such rules, but they look very natural to me)
Even more trivial, but: General prohibition of dropping in cartain board positions, even in these, to/from which it could normally move; Attacking any opposing piece, not only the King.
Sounds trivial, but I mention it: *) You cannot drop a piece onto an occupied square (all pieces in all drop variants I am aware of) Restrictions on Check/Checkmate: *) You cannot drop a piece giving checkmate (Shogi P)
I am looking for a compact way to describe the rules of Chess variants. For the moves of pieces (extended) Betza notation is ideal. But in variants with drops it does not tell you anything about which drop moves would be allowed, and which forbidden. So I am looking for a sufficiently general way to encode rules for dropping pieces. Pertty common restrictions are: *) You cannot drop a piece in a place where it can never have any legal moves (Shogi P, N and L, Crazyhouse P) *) You cannot drop a piece on a square where it could not otherwise go (Crazyhouse P) * You cannot drop a piece in a file where there already is one (Shogi P) * You cannot drop a piece in a file where there already are two (Tori Shogi Swallow). Do there exist any other variants that have special requirements for drop rules other than those listed above, that a general description method should be able to handle?
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