In support to blog post

RetainCycle6.swift

class Person {
    ...
}

class Apartment {
    ...
    weak var person: Person?
    ...
}

var person: Person? = Person(name: "John Doe")
var apartment: Apartment? = Apartment(number: 123)
person?.apartment = apartment
apartment?.person = person
person = nil
apartment = nil

// output:
// John Doe is being deinitialized
// Apartment 123 is being deinitialized