pub fn union<'a>(&'a self, other: &'a BTreeSet<T>) -> Union<'a, T>
    where
        T: Ord,
    {
        Union(MergeIterInner::new(self.iter(), other.iter()))
    }
Returns `true` if the set is a subset of another,
i.e., `other` contains at
benchmarks/haystacks/code/rust-library.rs