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Should I be able to use $dest to get the destination square before HandleMove runs?

I suppose not, because to work with betza.txt the preset has the "do not add moves" checkbox ticked, so that Game Courier does not handle the move itself before it invokes to the Post-Move code. And I suppose $dest would be set during GC's handling of that move.

I am not even sure this variable would be set after HandleMove returns. At some point HandleMove passes the move to GC with the aid of an "eval MOVE: xxx" statement, but I am not sure whether this is equivalent to latting the move be handled automatically.

But HandleMove has its own variables to describe the move: #ori, #desti, #moved and #victim, which will be set after HandleMove returns. And for side effects: #promo, #suicide, #freedrop and #dropped. When not applicable the first three will have value 0, and #dropped will be undefined if #freedrop is invalid.

Anyway, HandleMove true starts with calling ParseMove true. This extracts the values for all these parameters in so far these were explicitly specified by the move notation. Implide side effects are then derived by generating all moves of the indicated piece, and match those with the move parameters that were given in the input notation. If there is a match it then takes the additional parameters from the generated move (such as #suicide when the move was an e.p. capture).

So in a code section where HandleMove is not useful (e.g. because you are in the prelude), and you don't feel like parsing the 'move' that the player entered yourself, you could call ParseMove if the syntax of what the player was supposed to enter is compatible with that of normal moves.