#![allow(unused_variables)]
fn main() {
    use std::collections::BTreeSet;
    let mut books = BTreeSet::new();
    books.insert("A Dance With Dragons");
    books.insert("To Kill a Mockingbird");
    books.insert("The Odyssey");
    books.insert("The Great Gatsby");
    if !books.contains("The Winds of Winter") {
        println!("We have {} books, but The Winds of Winter ain't one.", books.len());
    }
    books.remove("The Odyssey");
    for book in &books {
        println!("{book}");
    }
}