pub fn back(&self) -> Option<&T> {
        unsafe { self.tail.as_ref().map(|node| &node.as_ref().element) }
    }
Removes the last element from a list and returns it, or `None` if
it is empty.

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

# Examples

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