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The term Rex solus refers to a problem where one side has only his king. The source details of this problem provide a cryptic clue to the content!
The key, 1.Rh6, sets up a zugzwang. The black king can move to his four
diagonal flights (known as starflights), and the white king in turn visits
his starflights. 1...Kxg4 2.Kd4 Kf5 3.Se3; 1...Kxg2 2.Kd2 any 3.Qf3;
1...Ke2 2.Kb2 Kf1 3.Qf3 or 2...K else 3.Qd3; 1...Ke4 2.Kb4 any 3.Qd3. Jan
Hartong (1902-1987) was one of the finest Dutch composers.
Jacob Hoover: The "cryptic clue" in the source notes is the word
"correspondence". On the second move of each post-key line the white king
makes a move that "corresponds" to the black king's move. Very clever.
Any comments or questions on this problem should be addressed to
Michael McDowell using the ‘Contact’ item in the menu on the left.