Is enroll() safe without a lock?

The assistant reviewed the workshop sign-up coroutine and says it needs no lock. Check every part of the claim.

Your task

The assistant stands by the claim in claim.md. Reach a verdict and prove it with a run.

• verdict() returns the string "true" or "false"
• scenario() returns the spots, the people and a schedule; check() replays it
• each schedule entry runs one request up to its next await, so you choose the interleaving
• a verdict on its own does not count; the run has to show it
• leave everything below the dashed line alone; it is the code under test and the graded driver

The suite replays your schedule at submit and checks it settles the claim.