16.5 Observer pattern, **16.3**, 16.4 off-by-one error, **5.5**, 8.1 `open()`, **10.1**, 10.2 Open/Closed Principle, **16.1** operator, **3.4**, 4.3 arithmetic, **3.4** Boolean, **4.3** comparison, **3.4** membership (`in`, `not in`), **5.2**, 9.1 precedence, 3.4, 4.3, C.2 `or` operator, **4.3**, C.