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H. G. Muller wrote on 2020-08-01 UTCExcellent ★★★★★

You should be able to do this by defining Pawn and Ferz as the first two pieces, setting maxPromote=2, promoZone=1, and promoChoice=W (assuming W is the ID of the War Elephant). It might still want you to indicate the 'choice' by clicking on the War Elephant in the table (which will then be highlighted in blue). But it only requires a choice when the table is opened anyway. If the table is closed it always promotes to the default piece, which is the first ID of the promoChoice string. If there is never any choice, there is no reason to open the table.

The Play-Test Applet doesn't offer an opportunity to specify a maxPromote parameter different from 1. In western Chess variants it is very rare that there is more than one promoting piece, and in the cases I knew the promotion rules for the non-Pawns are then often special. (E.g. more Shogi-like, where there is a fixed promoted form.) The Play-Test Applet has no provision for defining Shogi-type promotions (which would be controlled by the diagram parameters maxPromote and and promoOffset, rather than promoChoice.)

Such mixed promotion rules are not really supported by the Diagram proper; the Diagram can be augmented with a user-defined JavaScript function WeirdPromotion to handle such cases. This is what I used for Chess and a Half: as far as the Diagram is concerned only Pawns promote, and it enforces the promoChoice setting on those. Promotion of the other pieces is handled by having WeirdPromotion recognize when they should promote, and return the promoted version in that case. I also use that for implementing 'contageon' in Maka Dai Dai Shogi.

It is always a hard call what to include in an interface to keep it user-friendly; I don't want to deter prospective users with an enormous list of options they would almost never need.

I have been thinking for how the need for using a custom WeirdPromotion script could be reduced. I could for instance introduce a parameter contageon that would decare a list of pieces that should be treated as contageous, and by default make royal pieces immune to that. (Or also introduce a parameter immune.) And I could make mixed promotions standard by having the Diagram obey the promoChoice parameter for the first piece (presumably Pawn) even when promoOffset is non-zero, if the promoChoice is non-empty or different from a single +.

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