turton.cql

The CQL file turton.cql shows how to find examples of the Turton theme. The Turton theme is a beautiful problem theme that we illustrate here with an endgame study by one of the CQL developers: Costeff 2015, after 4...Rf4
(found from CQL file: turton.cql)

There are two thematic pieces to a Turton, which we call T and U. These must be line pieces, a ♕, ♖ or ♗ that can slide along the same direction (so that ♖♗) are disallowed. The idea of the Turton is that white wants to double T and U so that T will support U after a move by U. In order to arrange this doubling, T passes over a critical square C and then allows U to move to C. Had U just moved initially to C, then T would move forward supported by U. The idea of the Turton is that T and U switch orientations so that U can move forward.

In the diagramed position T is the ♕e3; U is the ♖a2; and C is e2.

White would like to check black on e8 with T or U, supported by the other. The immediate 4.♖e2 fails:

after 4. Re2? (variation)
(found from CQL file: turton.cql)

due to 4...♜×f7, when 5.♕e8+ fails due to 5...♜f8.

Thus, White can only win by switching the orientation of the ♕/♖ pair. White does this in a surprising way: 4.♕e7! ♛d1 reaching this position:

Costeff 2015, after 4...Qd1+
(found from CQL file: turton.cql)

Now white begins the main Turton theme with the critical move by T: 5.♕e1, crossing the critical square C:

after 5.Qe1
(found from CQL file: turton.cql)

Suprisingly, 5.♕×e1 6.♔×g2 wins here for white, so black tries to retreat with 5.♕d8 (stopping the back rank mate):

after 5....Qd8
(found from CQL file: turton.cql)

Now white moves U to the critical square C, completing the reorientation of T and U:

after 6. Re2
(found from CQL file: turton.cql)

Black tries his first defense with ♖×f6 but this now fails to 7.♖e8+ because U (the ♖ on e8 is supported by T, the ♕e1:

after 7. Re8+ 1-0
(found from CQL file: turton.cql)

turton.cql line-by-line

The first line simply guesses values for T, U and C. T will be a piece variable that is one of ♕♖♗:

    ◭T∊♕♖♗

T is a piece variable (like the piece variable in explain-likeinterference.cql) whose value is the square on which the selected piece lies. This represents the supporting piece in the Turton theme, one of ♕♖♗. U is another piece variable, the supported piece. The ∩~T insures that U is a distinct piece from T, since ~T are all the squares other than the one T is on. C is the critical square, an ordinary square variable. It can range over all the empty squares initially attacked by U, that is, □←U.

Once candidate assignments for T, U, and C have been determined, the main work of the CQL file begins. The

⊢T∪U

tells the ⊢ only to consider moves by the thematic pieces T and U.

The first constituent of the path indicates that the thematic T first moves to an empty square:

T――□

The next constituent is a filter constituent

T→C→from

This filter is evaluated in the position after T makes its first move (to an empty square, □). The value of from is the square from which T moved. The chained → indicates that that the three squares T, C and from are in a line, with the critical square C being attacked by T. Since T is as line piece, this means that T has crossed over the square C as if moved from from to its current square.

The comment

///"T crosses critical square " C

inserts the specified comment into the PGN file as a comment to the critical move. Note that the C in the comment is replaced by the actual square's name.

The next move of the theme is

U――C

This indicates that U moves to the critical square. There can of course be any number of moves by non-thematic piece in between these first two thematic moves. Finally, U moves to a square where it is supported by T through the now-empty critical square C:

U―― T→C→U

summary

Using piece variables and ――◎ can allow a concise description of the key ideas of a Turton theme, including the motion of thematic pieces and the concept of a critical move that crosses a critical square.