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The expression is "a b". It would evaluate to an array containing the two elements.

But why would it evaluate to (b a) rather than (a b)?

The match operator is multiple-arity.

Ah, now I am starting to understand it. Since #protected was supposed to be an (empty) array, match should have been ternary, and the expression would be OK. (And in fact it is). But because the initialization set protected ( ); did not make it an array at all, the problem you now point out occurred, and things became a mess. So fixing the initialization of protected (or actually having piece types in there) solved the problem.

I think what you want is this:

set traded cond and == #victim #protected == #mover #protected #mover 0;

That would have been what I wanted if #protected would have been a single piece type rather than an array of possible piece types. This code is for enforcing Chu-Shogi Lion-trading type rules, and even in the simple case it would contain both the white and the black type, to make the code color-independent. But there could potentially be multiple types that you cannot trade for each other (e.g. in Tengu Dai Shogi). Of course in almost every CV protected would be an empty set, but I figured that match on an empty set would not be very expensive. Perhaps this, and similar weirdness (such as the counter-strike rule, and iron pieces) should all be put in a conditional code section the execution of which is controlled by a single boolean. Which would then be set to false to skip the code by default, and only variants that have one of these exotic rules could then initialize it to true. I have no idea whether the code I have now would cause any significant CPU load; perhaps I should use the stopwatch trick to time the execution from Pre-Game to Post-Game.

For now I highlight all pseudo-legal moves without testing any for full legality, and it is having to do the in-check test on all possible moves that is by far the most expensive piece of code. The mate test only becomes expensive when you are in check; during most of the game the first move you try would already be legal, proving there is no mate.