Macro libsugar::btset

macro_rules! btset {
    { } => { ... };
    { $($e:expr),* $(,)? } => { ... };
}

new a BTreeSet<V>

btset![a, b]

equivalent to

{
    let mut s = BTreeSet::new();
    s.insert(a);
    s.insert(b);
    s
}