pub fn clear(&mut self) {
        *self = Self::new();
    }
Provides a mutable reference to the back element, or `None` if the list
is empty.

# Examples

```
use std::collections::LinkedList;

let mut dl = LinkedList::new();
assert_eq!(dl.back(), None);

dl.push_back(1);
assert_eq!(dl
benchmarks/haystacks/code/rust-library.rs