The phrase refers to a "Solution Guide" (GDZ) for the 10th-grade Russian language textbook authored by Vera Babaitseva.
The clock on the wall ticked toward 1:00 AM, the sound echoing in the silent kitchen. On the table sat the thick, blue-and-white textbook, open to a daunting chapter on complex syntax and punctuation in compound sentences.
With a newfound clarity, Misha closed the browser. He picked up his pen and began to rewrite his exercises, finally understanding where the commas belonged. The stress that had been tight in his chest began to loosen. Tomorrow's test was still coming, but for the first time all night, he felt ready to face it.