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Sensing that his passengers didn’t like speeding through the dark, Mandelbrot turned on the headlights. In their illumination Derec saw robots moving aimlessly along the sidewalks, as if unaware that anything had happened only a few blocks away.

“Good grief,” Derec said. “Don’t they care that half the city has been destroyed?”

Avery shook his head. “No curiosity, and they haven’t received orders. Why they haven t is a mystery, but it’s obvious they haven’t.”

As they drove on through the city, though, they began to notice more and more robots moving purposefully. “Looks like your new supervisors are getting things going again,” Avery said.

Even as he spoke, the lights came back on. In the sudden brilliance, Derec nodded his agreement. “Looks like,” he said. He twisted around in his seat and looked back the way they had come. A dark wedge still cut into the city’s glow. He wondered how long it would take to erase that scar. In a normal city it would take years, but here? Maybe a day. Two at the outside.

The Compass Tower was the first building erected in a new robot city, and the only building to remain unchanged from day to day. As such, it housed the city’s central memory, served as communications center, and also became a general meeting place. It was no surprise to find all seven of the city’s supervisor robots there, nor, judging from the comlink static, to find them all standing immobile in the main conference room, locked in communication fugue. The three experimental robots were there as well.

This conference room was not a windowless closet. It was near the top of the building and had windows on three sides looking out over the city. Avery stood in the doorway a moment, surveying the scene, then raised the cutting laser up to aim at Lucius.

“Are you sure you want-?” Derec whispered, but Avery had already fired.

A shower of molten metal erupted from the robot’s chest. Avery moved the point of destruction upward, toward its head and the positronic brain contained within, but the beam never reached its mark. Threatening Derec with a laser hadn’t been enough to bring Lucius out of communication fugue before, but now that it was his own body under fire, Lucius became a blur of motion; a window suddenly grew a robot-sized hole in it, and he was gone.

Avery flicked the beam toward Adam, but he and Eve had already begun to move. Two more crashes and they were gone as well. Derec and Avery ran to the window in time to see three gigantic bird shapes disappear around the edge of the building.

The supervisor robots had also awakened, but they made no move to escape. Avery turned away from the window to face them and said, “All of you, deactivate. Now.”

Six supervisors froze in place. The seventh took a halting step forward, said, “Please, I must-”

Avery fired his laser, this time at the head instead of the chest. The robot fell to the floor, showering sparks. Avery swept the laser over the others, heads first, then methodically melted them all into puddles. When he was done he turned to the four robots Mandelbrot had brought with him. “You four are now supervisors. Access the central library for your duties.”

“Yes, Master Avery,” they said in unison. They were still for a moment, consulting the library via comlink, then as one being they turned and left the room to begin their new jobs.

Something about the sight sickened Derec. Seven cooling puddles of recently free robot stained the floor, and four new slaves moved off to take their places. And yet, and yet, what else could Avery have done? They had seen what happened when supervisors failed to perform their duties. The old supervisors might have been still usable-the one who had defied Avery’s order might have been about to protest that he must see to restoring the city-but who could know? If that hadn’t been what he’d been about to say, and if Avery hadn’t fired when he had, they might have had ten renegade robots on their hands instead of three.

Three were bad enough. Time and again throughout the night, reports came in of the robots attempting to distract others from their duties. Avery had ordered hunter-seekers out to stop them, but that merely stopped the problem wherever there were hunters. He and Derec considered the idea of ordering all the city robots to arm themselves against the renegades, but rejected it after only a moment’s thought. One didn’t arm the peasants during a revolution.

Derec and Mandelbrot went back to the apartment and brought Ariel and Wolruf back to the Compass Tower, reasoning they would be safer there, guarded by hunter-seekers whose definition of “human” had been strengthened and refined to include the tower’s four organic occupants, no matter who said otherwise. While he did that, Avery worked to strengthen the definition of human for all the city’s robots, and thus the Second Law compulsion to obey.

Avery was a virtuoso at the computer. By the time Derec returned, he had finished the reprogramming and had even discovered the sequence of events that had led to the building’s collapse.

“Look here,” he said, motioning Ariel and Wolruf over to look at the screen as well. “I’ve got it displaying a priority map. This, down here at the bottom, is the original city programming.” He pointed to a layer of blue near the bottom of the screen. Tiny blue lines rose from it to the next level, a green layer; some passed on through. “These lines are orders. The next level here, the green, is what you three put in when you were here last. Notice how your program stops nearly all the orders from the original layer. That’s because you told the robots to quit expanding the city and to become farmers. They had a completely new instruction set. But look here.” He pointed to a thick blue line extending up through the green layer. “You left in the part that lets the city metamorphose at random. Not a problem, but now this layer above that, the red one, is what the aliens-these Ceremyons of yours-put in. It’s basically an order to ignore all the ‘do’ instructions in your level of programming, but keep all the ‘don’ts.’ See how every green line stops at the red boundary? All that gets through is the basic city maintenance that you left in, including the random metamorphosis. It worked just fine as long as the supervisors were in the circuit, because they also had verbal orders to keep things running, and they had enough volition to order things that weren’t automatic anymore, but as soon as you take them out of the circuit, the whole thing falls apart.”

He turned away from the screen and spoke directly to them. “So here’s what happened: Building movement is essentially random, subject to supervisory override if the random number generator comes up with a ridiculous configuration. It doesn’t happen often, about once a day, on the average. So without a supervisor to veto it, today’s ridiculous building gets built. It turns out to be ridiculously tall. But the main power station doesn’t have a supervisory order to generate more power for it, so when it starts to pull excessive current to lift all that mass, it trips the breaker. Power goes out. The original emergency programming has been blocked-twice, I point out with injured dignity-so without a supervisor’s order, the auxiliary stations don’t go on line. The building is unstable without power to hold it up, so it falls over. On the power station.”

“Oh,” Derec said. One word can be expressive under the right circumstances.

Ariel said, “So we messed it up, that’s what you’re saying? It’s our fault?”

Avery shook his head. “It’s everyone’s fault. Mine for not writing the original program to filter out the bad input before it reached the supervisors, yours for bypassing the emergency programming, the Ceremyons’ for bypassing your bypass, the experimental robots for distracting the supervisors-take your pick. We’ re all in this together.”