In support to blog post

RetainCycle3.swift

class Person {
  ...
}

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

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

person = nil

// output:
// John Doe is being deinitialized