#[derive(Debug)]
enum A {
    B(B),
    C(C),
    A1,
    A2,
}

#[derive(Debug)]
enum B {
    B1,
    B2,
    B3,
    B4,
    B5,
}

#[derive(Debug)]
enum C {}

fn main() {
    let a = A::B(B::B5);
    match a {
        A::B(B::B5) => println!("success"),
        A::B(B::B4) => println!("success"),
        _ => println!("error: {:?}", a),
    }
}