Concurrent thin documents allowed, which can break memory safety

[shepmaster]

fn double_thin_document() {
    let p1 = Package::new();

    let (s1, mut c1) = p1.as_thin_document();
    let (s2, mut c2) = p1.as_thin_document();

    for _ in 0..10 {
        let c = s1.create_comment("hi");
        c1.append_root_child(c);
    }

    for c in c1.root_children() {
        let e = s2.create_comment("bye");
        c2.append_root_child(e);

        println!("{:?}", c);
    }
}

[shepmaster]

idea: as_thin_document() requires &mut ?