Harry, or rather, Harry's internal Richard Feynman, began visualising numbers going by on a tape.

The second volume of Feynman's autobiography had a story in it about counting up to a minute, and like the real physicist, Harry's internal Feynman consistently undershot.

Regardless of anything else, Hufflepuff started, we're saving as many people as we can.

There are a lot of things we know that look like more than one thing, on the object level, Ravenclaw pointed out.

From the base of Harry's wand, an almost invisibly thin black line crept up onto the heel of his hand and down the underside of his forearm as he reminded himself, over and over again, that the entire definition of the cornified layer of the epidermis was the part of it that was already not alive.

His internal Feynman held up ten fingers.

Partial Transfiguration could in fact look like a bevy of different powers - cutting, lubricating, joining, drilling - if you hadn't already seen through the convenient manmade categories of physical form to the raw yet completely conceptual factors in the wavefunction whose factors you were modifying. Could you perform partial Transfiguration without understanding quantum mechanics? He doubted it. But even if he couldn't buy back multiple hostages with it, he could buy time, and that would at least help.

All right, everybody, snapped his Gryffindor, stepping up as field marshal. What do we know that he doesn't? Partial order, rank by existential risk from disclosure.

Patronus 2.0 at the meet of the lattice, said Ravenclaw. He's seen it already, he's got to know no ordinary Patronus could bring somebody back to life, the Death Eaters won't understand what we're saying unless they speak Parseltongue, and in any case neither they nor Voldemort can learn it. But he'll see the utility in a spell that children can learn that can reverse death.

Also the fact that Draco can cast a Patronus, Slytherin added. Dangerous to Draco, unless he counts it as a secret worth protecting Draco for.

Feynman held up two fingers, then ten fingers.

The fine black line coursed down Harry's back, toward his leg, as a Form continued to take shape in his mind.

The hypergraph that characterised Harry's memory blazed with activity as he sieved through everything that might be a candidate that could satisfy Voldemort's specification. Most candidates fell hors du combat for one reason or another - something Voldemort already knew, or that fell into the far broader category of Muggle arts - though he deliberately cached a few losers he thought he had a chance to work in opportunistically to pad the conversation. Alongside the Form, a lattice took shape, tracing out a map of options with terminating conditions above and below, related to one another - or not - in terms of how much harm Harry figured Voldemort could do with them. If Voldemort did survive their encounter, there was good reason to buy what lives he could save as cheaply as he could.

He had to keep the Form in his mind, though, no matter what, to re-braid the topological strands of the universe to call it into being. And he might have to do that while simultaneously re-braiding different strands to call a beginner's lesson for Voldemort into being. And keeping enough attention in reserve to be able to guide Voldemort's efforts to learn.

That meant having to maintain the Form on automatic while still being able to talk. In Feynman's informal experiment, when he counted aloud in his head, he could read but not speak, while his colleague, who visualised numbers running by on a tape, could speak but not read. That suggested that anything that didn't empirically get in the way of keeping Voldemort talking was a fair target for mindworming himself with the Form.

To do anything to them, he needed a physical connexion to them. Thirty-six Death Eaters plus Voldemort meant thirty-seven parallel connexions.

There was a name for a circuit that connected one input to dozens of parallel outputs: a bus.

Sixty, announced his inner Feynman. It was time to start talking.

The nanocarbon tubes go down, down, down...

Oh, of all the stupid earworms- Harry's Inner Critic started.

Slytherin cut him off with a hiss. If it works, it wasn't stupid.

The fine black line reached Harry's heel, drilled several inches into the soil, and branched two hundred and fifty-six nanocarbon wires in thirty-eight underground directions, because Hermione was also a useful direction and nobody ever died from too much redundancy.

"Sspell that resstored life and magic to girl-child's body is not sstandard-isssue Patronusss," hissed Harry. He wasn't sure how many seconds he had left in the minute - not many - but his inner Feynman started counting on his tape again. Five times through was close enough to four minutes that he could weigh crude estimates of how much time he'd stalled for against how much Transfiguration he had left to do.

The wires on the bus go through the ground, through the ground, through the ground...

"Obviousss already." Voldemort lifted just the tip of his wand, like a conductor about to lead his orchestra. "Alsso usselesss. Even could I masster, cannot ressurrect sself if already dead."

"Not you, nor Death Eaterss." Harry glanced at the semicircle, visually orienting the network that was steadily growing underground toward them, then back to Voldemort (and beyond him, the altar) and hoped it looked conversational. "But you have hosstagess, sstudentss, who can learn. Can devise incentivess to keep ssuccessful ones loyal." He was fairly sure that was true, anyway; not only had he said it in Parseltongue, he could cast the True Patronus and Voldemort seemed to be doing a fine job manipulating him through incentives, as far as all the outcomes so far were concerned.

"Not sso sstupid after all, are you, boy." Voldemort paused, but for less than a second. "Sstill, you have not sshown me how to masster life-resstoration, only that it can be done. Tempting to keep you alive long enough to insstruct sservant ... but no, too risssky. Too many opportunitiess to fulfil prophecy in time it takes to teach."

Harry flinched and his breathing quickened. He focused just as much of his attention as he had to on looking scared while he gasped, just as, some part of him observed, the Harry Potter who might once have thought he could sway a Dark Lord from a precommitment with something like satisficing would have done. The greater portion of his attention borrowed the phase of the ginned-up rhythm for the loop that was generating his Transfiguration to lock onto, the WIRES on the BUS go THROUGH the GROUND...

Under the earth, the nanocarbon tubes reaching toward the Death Eaters began to converge on their individual targets in bundles of six. One twenty-wire bundle had already settled beneath where Voldemort hovered, and another of the same size was still crawling its way toward the altar where Hermione lay. Thirty-six six-bit words and two twenty-bit words, that's the weirdest addressing scheme- his Inner Critic began, before every one of Harry's other mental models, including ones with no reason to know what an addressing scheme was, turned to it in unison and chorused, "SHUT UP."

Also, sixty, said Feynman.

Harry fixed his gaze on Voldemort once more. If he was going to keep on buying time, it would have to be with bribes Voldemort would accept. Giving up Draco's secret would only be throwing him under the bus (Harry's inner Hofstadter quelled a chuckle) for no reason. He reached into the lattice and picked out a covering element he could raise the stakes with. "Ssame power iss ssource of power over life-eaterss."

Under the ground, far below where Voldemort hovered, a conical shape, one he had Transfigured once before from an ice cube, began growing downward from the end of the first Voldemort bus wire. Let's call that V0, he decided, even though V2 would have been so much more thematically appropriate.

"Ceasse trying my patience, sstupid child. There iss no pot ssweet enough to convince me to let you leave thiss place alive."

Right, then. Voldemort considered Harry's immediate-future continued existence to be of such negative utility that even ironing out the last bugs in full-on magical resurrection and making him invulnerable to Dementors didn't compensate for it. Harry had not expected that, but that was why he had planned for it.

Besides, on the gripping hand, there was only so much more time he needed to even the odds to a point where he felt safe expending attention on something as complicated as how to flip the sign on an ultimatum-game utility function Voldemort had already precommitted to, which was going to mean figuring out why his coefficient in it was so absurdly negative.

"Can insstruct in Art which ssecured esscape from life-eaterss' sstronghold. Iss ssubtle Art, ssaid by all to be imposssible. Headmasster sspeculated wass power Dark Lord knew not."

One by one, the tips of seventy-two nanocarbon wires touched the soles of seventy-two Death Eater boots and carried outward to an edge, then upward, fused with the leather as if they had always been there. D0-0 and D0-1 through D35-0 and D35-1.

"Sspeak sswiftly, then," Voldemort hissed. "But if usseless, your death beginss."

"Can Transsfigure only partss of objectss, without Transsfiguring them entirely. In metal fortresss, ssliced cylinder through wall, sslanted to collapsse insside, by Transsfiguring ssurface area to motor oil. Cancelled Transsfiguration once esscape route was open, left no trace of oil on ssurfacess, your sspell did the resst."

Voldemort was too far up for Harry to read anything into his expression, not that Harry expected to be able to determine anything reliable from the too-smooth face and uncanny-valley features. Harry just hoped the pause meant the Dark Lord was thinking. The wires on the bus pass through their robes...

Sixty, Feynman said, and started again.

"Ssatissfactory." Voldemort kept the gun trained on Harry, and remained airborne, but Harry's Gryffindor was turning cartwheels anyway. "Demonsstrate. Can obsserve perfectly well from here." He reached into his robes for a moment, then flung his arm wide, toward Harry. Something tiny flashed silvery in the moonlight, tumbling, then came to rest in front of Harry's left hand.

"Take Muggle artifact," Voldemort ordered. Harry obediently picked up the paperclip. "Are risskss of partial Transsfiguration ssame as full Transsfiguration?"

"To besst of my undersstanding."

"Then sshow me. Ssusstain Transsfiguration, when finisshed, and drop artifact on ground."

As deliberately as he could, since he was performing for an audience that could see where his wand was, had been instructed to kill him, and had not been explicitly told that Voldemort might tell him to use it, Harry lowered the paperclip to the tip of the wand and began to think. The paperclip was a simple Form, the bus much more complex, and the Most Important Thing In the World right now was making sure that the former didn't disrupt the latter on its way into existence.

The soundtrack in his head shifted momentarily. Their robes are connected to, my wand, 'cause IS-CONNECTED-TO is, transitive ... the robes on the bus go up and up...

Something about the Form of the bus contracted. There wasn't time for Harry to collar his working memory of what had just happened and interrogate it to find out what it was, but when he observed the Form in his mind now, he felt no seams where the wires had already laid themselves - just a single continuous pipeline, branching exactly as he willed it to, interconnecting with its peripheral environment solely by his leave. Only the objectives he had yet to gain carried any sense of being boundaries now.

One of those was the paperclip. For a moment he considered zebra-striping it - he could hijack the rhythm of his earworm to speed up the process - then decided it was best not to introduce Voldemort to the identical style he was using to construct the Form whose obscurity he was relying on Voldemort's ignorance of partial Transfiguration to preserve. The last thing he needed was Voldemort suddenly noticing the similarity between the partially Transfigured paperclip and something else already going on in the wild.

You could hear multiple things at once, and all sounds were waveforms, and a half-steel-half-wooden paperclip was a particularly simple superposition of waveforms. Harry closed his eyes. In his mind, gusts of wind, as if from multiple high-speed fans, began to howl. After a few seconds, he opened his eyes. The wider end of the paperclip no longer reflected the moonlight. He pressed a fingernail against it, and it gave, as was proper for thin wood. With his earworm still droning away, he cautiously began the process of Sustaining the transfigured paperclip. As he felt his working reserve of magic drop, the fan noises died away as if someone had hit a kill switch; the earworm carried on unabated.

Meanwhile, the last bundle of wires reached the altar, and Harry recognised the strand that began climbing it to inscribe a cursive message for Hermione into its surface as H0, rather than having to think of its name consciously. Same for H1 through H9, silently anchoring themselves to his belongings in the heap where Voldemort had left them.

Harry dropped the paperclip on the ground. Voldemort raised a hand, and the paperclip blinked away faster than Harry could follow. Above, Voldemort raised his hand and peered up, illuminating the paperclip between thumb and forefinger.

About that utility function, Harry realised.

"Occurss to me, teacher," he hissed. "Do not know whether Sstone affects partial Transsfiguration jusst as it doess whole. Ssimple enough experiment, though..."

It couldn't be enough, not yet. But with the bus in place and his options far more open, Harry felt a lot more comfortable loosing his grip on some of his attention, exploring a different graph now: Find me a solution space of incentives Voldemort isn't even considering, because he's playing the game at the wrong level...