struct Animal {
    int delegate(int) fly;
    void delegate() crawl;
}

// Use structs
struct Bird {
    Animal animal;
    alias animal this;

    Wing[] wings;

    this()
    {
        animal.fly = &flyImpl;
        anumal.crawl = &crawlImpl;
    }

    int flyImpl(int) { return use(wings); }
    void crawlImpl() { }
}

// Use closures

Animal makeBird()
{
    Animal bird;
    Wing wings;

    bird.fly = (int) { return use(wings); }
    bird.crawl = () {};
    return bird;
}