Rohan waited in line. A walk-in took his book.

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Rohan is first in line for Godan. A copy comes back. Get one step wrong, and a walk-in borrows it first.

The naive return: put the copy back, then tell the queue. In between, a walk-in at the desk borrows it. The member who waited loses his turn. The fix: return and hold in one locked step. The copy never touches the open shelf.

A copy's life: available, on loan, available again. But if someone's waiting, a return goes on hold instead. On hold is held for one member. It is not available. Only the waiting member can collect it.

20 November. Zoya returns G2. It's held for Rohan until the 23rd. Next day, Tara asks at the desk. No free copy. It's spoken for. Rohan never comes. On the 24th, his hold expires. It passes to Tara, next in line. She collects on the 25th.

Return and hand-over to the queue: one locked step. A copy on hold is not available. The person who waited gets it.

Full breakdown in the lecture.