struct Foo {
}

enum FooChain<'a> {
    Top(&'a mut Foo),
    Child(&'a mut FooChain<'a>)
}

impl<'a> FooChain<'a> {
    fn child(&'a mut self) -> FooChain<'a> {
        FooChain::Child(self)
    }
}

fn foo<'a>(chain : &'a mut FooChain<'a>) {
    chain.child();
}

fn main() {

    let mut a = Foo {};

    {
        let mut b = FooChain::Top(&mut a);
        foo(&mut b);
        foo(&mut b);
    }
}