This would be a little clearer if you asserted equality instead of printing, because I can't see what it prints without actually running the code locally. Instead you could use something like

```rust
assert_eq!(map.len(), 3);
assert_eq!(map.get("a key"), vec!["value", "new value"]);
asesrt_eq!(map.get("b key"), vec!["value", "new value"]);
assert_eq!(map.get("z key"), vec!["new value", "value"]);
```