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if capture and match $old (bl br):
    capture where $dest direction $origin $dest;
endif;

I put this in the post move, but when it runs it captures the piece at $dest rather than the specified location. It works correctly if I change it to

if capture and match $old (bl br):
    set x where $dest direction $origin $dest;
    capture #x;
endif;

but I don't see why it would make a difference