The file smotheredmate.cql matches positions with a smothered black mate: all black's king's escape squares are occupied by black pieces. If you want to include white smothered mates, run cql with the -flipcolor option:
  cql -flipcolor smotheredmate.cql.

Alternative, you can insert a before the beginning of the body and wrap the body in braces:

 {btm mate not 

smotheredmate.cql line by line

btm means black to move; mate of course means that black is thus in mate.

is the set of squares that empty; is the set of squares occupied by a white piece.

is the union of these sets of squares, that is, all the squares not occupied by a black piece.


is true if the attacks some white piece or empty square.



is true if the only attacks black pieces, hence is in smothered mate.