#![allow(unused)]
fn main() {
use std::collections::BTreeSet;

let mut a = BTreeSet::new();
a.insert(1);
a.insert(3);
a.insert(5);
a.insert(6);
a.insert(10);
a.insert(8);

let mut b = BTreeSet::new();
for i in 1..60 {
    b.insert(i);
}
let diff: Vec<_> = b.difference(&a).cloned().collect();
println!("{:?}", diff);
}