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