The Aegon Computer Chess Tournament is one of the most curious events in the calendar. Other tournaments may have one or two computers pitted against human opponents, but the Aegon tournament, now in its 10th year at The Hague, has enough of them to cause a traffic jam on the superhighway.

This year's event is the largest of the series with 48 machines, each running a different chess program, competing against 48 humans. And after four rounds, the machines are well ahead.

The turning-point came in round three when the building in which the event was taking place suddenly fell victim to a power cut. The machines all had to be stopped for half an hour while they were restarted and the current positions reloaded into their memories. Play then resumed with the machines apparently content but some of the players grumbling about having to play in conditions of poor light.

Had the machines conspired to overload the electricity circuits? Had they studied the Fischer-Petrosian match of 1971 in which the Armenian former world champion's play had gone to pieces after a similar power failure? Nobody was certain enough to make precise allegations, but there was little doubt that some of the human players felt they had been the victims of some mechanical gamesmanship.

In the first round, the humans had been beaten by a score of 28-20. But that was clearly due to the intimidating novelty of facing so many machines. In round two, human supremacy was reasserted with a 27-21 victory. But after the lights went out in round three, the machines won again by 27- 20 and the following day they scored an event more resounding victory by 301/2-171/2. With two rounds left to play, the machines lead by 1061/2- 841/2 and the leader board looks like something from a software catalogue: M-Chess Pro, Hiarcs and Quest lead with 4 points, half a point ahead of Chess Genius X, Mephisto Genius 68030, Schach 3.0, W-Chess, Tasc R 30, and John van der Wiel. Only the last of those nine leaders is a human. Even John Nunn, who has been one of the most computer- unfriendly opponents in recent events, has lost his unbeaten record.

Every human grandmaster knows how to play against computers: you get them out of their books, keep the position closed to avoid tactical accidents, then patiently build up an attack that is unstoppable by the time they see it coming. But putting those rules into operation under the stresses of tournament play and facing a remorselessly accurate, gormlessly uncomprehending and infuriatingly unemotional opponent, is never as easy as it sounds.

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Enough of artificial stupidity. Here's a problem in honour of Stephen Hendry. It's White to play and mate in four, and if you can't solve it try taking the white up to the baulk end of the table, then playing it off three cushions to get position on the black.

Solution: 1.c8=B!! b3 2.Bg4 b2 3.Bd1! Kxb1 4.Bb3 mate.

William Hartston