Mara and Alex traced the diagram back to a name embossed on the book’s inside cover: Dr. Elias Rhee — a man brilliant enough to make colleagues uncomfortable. Elias had disappeared from the academic circuit after publishing a paper that contradicted an industry standard. Rumor said he’d been right. Rumor said he’d been silenced.

In the end, the solution manual had been exactly what it promised on the cover — not only answers to exercises but a map for thinking, a template for turning pen-and-ink solutions into systems that actually held the world steady. And the city, by a slow, stubborn application of discrete controllers and communal will, learned that engineering’s truest solution was not a closed-form proof but a shared practice guided by clear math and a refusal to hide what kept people safe.