pub fn front_mut(&mut self) -> Option<&mut T> {
        unsafe { self.head.as_mut().map(|node| &mut node.as_mut().element) }
    }
Appends an element to the back of a list.

This operation should compute in *O*(1) time.

# Examples

```
use std::collections::LinkedList
benchmarks/haystacks/code/rust-library.rs