pub fn or_insert(self, default: V) -> &'a mut V {
        match self {
            Occupied(entry) => entry.into_mut(),
            Vacant(entry) => entry.insert(default),
        }
    }
Provides in-place mutable access to an occupied entry before any
potential inserts into the map.

# Examples

```
use std::collections::BTreeMap;

let m
benchmarks/haystacks/code/rust-library.rs