The Library Register
Scenario the First
From the day-book of the keeper.
How to read this register. Each cell may be marked with a ✓ (it is so) or a ✗ (it cannot be). Click a cell to cycle it — blank, then cross, then check. Right-click, or shift-click, to cycle the other way.
Read the marginal notes at left. Eliminate what cannot be; what remains must be. The register balances when every row and every column bears exactly one ✓. When that is done, check the ledger below.