pub fn back_mut(&mut self) -> Option<&mut T> {
        unsafe { self.tail.as_mut().map(|node| &mut node.as_mut().element) }
    }
Splits the list into two at the given index. Returns everything after the given index,
including the index.

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

# Panics

Panics if `at > len`.

# Examples

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