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836134202e
The existing test can be solved with the following:
```rs
while let Some(integer) = optional_integers.pop() {
assert_eq!(integer.unwrap(), range);
```
Similarly with `expect(...)`, `unwrap_or(0)`, `unwrap_or_default()`, etc. However, none of these solutions use the learning point of stacking `Option<T>`s.
The updated test can _only_ be solved by stacking `Option<T>`s:
```rs
while let Some(Some(integer)) = optional_integers.pop() {
assert_eq!(integer, cursor);
```
With the updated test, using `unwrap` or `expect` will panic when it hits the `None` value, and using `unwrap_or` or `unwrap_or_default` will cause the final `assert_eq!(cursor, 0)` to panic.
Exercise to Book Chapter mapping
| Exercise | Book Chapter |
|---|---|
| variables | §3.1 |
| functions | §3.3 |
| if | §3.5 |
| primitive_types | §3.2, §4.3 |
| vecs | §8.1 |
| move_semantics | §4.1-2 |
| structs | §5.1, §5.3 |
| enums | §6, §18.3 |
| strings | §8.2 |
| modules | §7 |
| hashmaps | §8.3 |
| options | §10.1 |
| error_handling | §9 |
| generics | §10 |
| traits | §10.2 |
| tests | §11.1 |
| lifetimes | §10.3 |
| iterators | §13.2-4 |
| threads | §16.1-3 |
| smart_pointers | §15, §16.3 |
| macros | §19.6 |
| clippy | §21.4 |
| conversions | n/a |