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Thank you, that helps. I think there might be another problem with what I was trying also.

map nw -1 1 n 0 1 w -1 0 e 1 0 s 0 -1 se 1 -1;
print mergeall where #0 n where #0 e;
print mergeall where #0 0 1 where #0 1 0;

When I run this, the output is

Array
(
)
Array
(
    [0] => b1
    [1] => a2
)

so it seems like the logical directions don't work in this context. I've also noticed that there are several "logical directions" links on this page that don't go anywhere.

I also have an unrelated question. Is there any way to control which space is highlighted after a move is made?