pub fn contains(&self, x: &T) -> bool
    where
        T: PartialEq<T>,
    {
        let (a, b) = self.as_slices();
        a.contains(x) || b.contains(x)
    }
Removes the first element and returns it, or `None` if the `VecDeque` is
empty.

# Examples

```
use std::collections::VecDeque;

let mut d = VecDeque::new();
d.push_back(1);
d.push_back(2);

assert_eq!(d.pop_front(), Some(1));
assert_eq!(d.pop_front(), Some(2));
assert_eq!(d.pop_front
benchmarks/haystacks/code/rust-library.rs