smotheredmate.cql

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.

Thus,

♚→□△

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

Hence,

 not ♚→□△

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