fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("DrainFilter").field(&self.inner.peek().map(|(k, _)| k)).finish()
    }
Returns the symmetric difference of `self` and `rhs` as a new `BTreeSet<T>`.

# Examples

```
use std::collections::BTreeSet;

let a: BTreeSet<_> = vec![1, 2, 3].into_iter().collect();
let b: BTreeSet<_> = vec![2, 3, 4].into_iter().collect();

let result =
benchmarks/haystacks/code/rust-library.rs