The file mirrormate.cql matches positions with a mirror mate: the mated King is surrounded by 8 empty squares.

mirrormate.cql line-by-line

The body of mirrormate.cql consists of a single filter. This filter a single argument, which in this case is the compound filter:
This compound filter is true only if each of its three constituent filters are true; together, these three filters describe a black mirror mate. The btm means black to move; mate means black is in mate.

The key filter in the compound filter tests that the attacks 8 empty squares. Here, the filter

is true if the attacks some empty square ( is the set of empty squares; means attacked by).

But this filter has a value.

The value of is the set of empty squares attacked by the .

To test that this set has 8 elements, we use == to compare it to 8, just like we did for the piece designators in explain-QQqq.cql.

Thus, the compound filter in the body of the cql file matches any black mirrored mate.

The effect of

When is placed in front of a filter, it matches positions that match either the original filter or the original filter with all its colors reversed. For instance, btm would be changed to wtm and a2 would be changed to a7. Here, the compound filter with reversed colors is:
This new compound filter matches a white mirror mate. Thus, the full CQL body now matches either a black mirror mate (the original compound filter) or a white mirror mate (the reverse color compound filter). That is, it matches a mirror mate.