No, of course you would maintain a Set<Customer> as well (possibly even a Map<Customer, Account Set> for efficiency).

... if you model it that way you're not using cyclic dependencies at all and this is in fact exactly how you would model it in a functional way.

Their whole premise is that

> Customers rely on Accounts (I.e. have one or many).

I'm giving a domain relevant counter example to that premise.

And interestingly enough, that leads you to a clean way of modeling it without the cyclic dependency as you've just done.