“Homework,” Elias said, smiling. “Exercise 17. Corrected.”

“See?” Caron growled. “The sequence step 5 never activates. It’s a ghost.”

“You don’t replace the ghost,” Elias whispered to himself, reading the professor’s note in the PDF. “You filter the signal.”

The other electricians nodded. They had spent weeks replacing sensors, relays, and even the entire PLC rack. Nothing worked. They had reached the limit of trial-and-error.

Elias didn’t say a word. He pulled out his scratched-up tablet and opened a file he had downloaded the night before from a forgotten university server. The file was called: .

Silence.