#![allow(unused_variables)]
#![feature(btree_set_entry)]
fn main() {
    use std::collections::BTreeSet;
    use std::collections::btree_set::Entry::*;
    let mut singles = BTreeSet::new();
    let mut dupes = BTreeSet::new();
    for ch in "a short treatise on fungi".chars() {
        if let Vacant(dupe_entry) = dupes.entry(ch) {
            match singles.entry(ch) {
                Vacant(single_entry) => single_entry.insert(),
                Occupied(single_entry) => {
                    single_entry.remove();
                    dupe_entry.insert();
                }
            }
        }
    }
    assert!(! singles.contains(&'t') && dupes.contains(&'t'));
    assert!(singles.contains(&'u') && ! dupes.contains(&'u'));
    assert!(! singles.contains(&'v') && ! dupes.contains(&'v'));
}