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Larry,

I believe I have code in which the cond operator is working properly. For example, it is used in 

http://play.chessvariants.org/pbm/includes/ivorytower.txt

and the code for Storm the Ivory Tower works. It is used in the code for Clodhoppers (C/c), for example, and in test moves I've made, only legal moves have been accepted.

Cond is a useful operator for pieces with divergent moves, such as, for example, pieces that move one way and capture another way, or pieces that move differently depending on their position. Cond can be used to check which rules of movement apply to a piece's move and then to apply the correct rule of movement. To illustrate, I'll give an analysis of its use in this line of code:

def C cond cond empty #0 (not capture) (empty #1) (checkride #0 #1 1 1 or checkride #0 #1 0 1) (checkhop #0 #1 1 1 or checkhop #0 #1 0 1) and fn legaldir #0 #1;

Remember that evaluation begins at the end and proceeds backwards. First, it checks whether the move was made in a legal direction. This is a requirement for any move in this game, and it is not part of the cond statement. It just stops the move if it is in a direction no arrow points.

This code has two cond statements. The inner cond statement, which sets the condition for the outer cond statement, checks whether the move was not a capture. I think I expressed it as a cond statement to allow the function to be used for hypothetical moves that get evaluated during the stalemate function. In a normal move, the origin space (#0) will not be empty, and it will just need to evaluate (empty #1) to tell whether the move was to an empty space (and hence not a capture). In a hypothetical move, the origin space might be empty, though my memory is hazy on this, and I can't pinpoint why this would be right now. Anyway, this inner cond returns true if the move was not a capture and false if it was. So, if it was not a capture, it evaluates whether it was a legal riding move, and if it was a capture, it evaluates whether it was a legal hopping move, because Clodhoppers capture by hopping but otherwise move by riding.