#![allow(unused_variables)]
#![feature(btree_cursors)]
fn main() {
    use std::collections::BTreeSet;
    use std::ops::Bound;
    let set = BTreeSet::from([1, 2, 3, 4]);
    let cursor = set.upper_bound(Bound::Included(&3));
    assert_eq!(cursor.peek_prev(), Some(& 3));
    assert_eq!(cursor.peek_next(), Some(& 4));
    let cursor = set.upper_bound(Bound::Excluded(&3));
    assert_eq!(cursor.peek_prev(), Some(& 2));
    assert_eq!(cursor.peek_next(), Some(& 3));
    let cursor = set.upper_bound(Bound::Unbounded);
    assert_eq!(cursor.peek_prev(), Some(& 4));
    assert_eq!(cursor.peek_next(), None);
}