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BDS & Michael McDowell

1st HM., Problemesis, 2003

#2
SIRENs e5; h8:
TRITON a3:
NEREID c3

1.Sa4!      (2.SIg7#)

1...SIh2     2.SIg3#
1...SIh5     2.SIg5#
1...SIb8     2.SIc7#
1...SIe8     2.SIe7#
1...SIg7     2.SIxg7-h8#
1...TRxa4-a5 2.NDb2#
1...e2       2.SIc5#

The key guards c5, allowing the white siren to unguard it when playing the threat. With the threat, which is a battery mate from the wNDc3, White has to stop the bSI from occupying e5, the square the wSI has just left. Only 2.SIg7 does this. Black's first four variations move the bSI so it can approach e5 from a different direction, but each time White can repeat the idea in the threat and move the wSI next to the bSI, again stopping it from going to e5. 1...SIg7 leads to a simple recapture; 1...TRxa4-a5 captures the key piece, so removing its new guard, but by doing this it gives up its control of c3, allowing 2.NDb2 which is mate from e5. This is a change of roles between the pieces in the battery. Finally the flight-granting 1...e2 leads to 2.SIc5#.