Using a generic class for multiple types

We can create instances of the Party<T> class by replacing the T generic type parameter with any type name that conforms to the constraints specified in the declaration of the Party<T> class. So far, we have three concrete classes that implement both the AnimalProtocol and Equatable protocols: Dog, Frog, and Lion. Thus, we can use Dog to create an instance of Party<Dog>—that is, a Party instance of Dog objects.

The following code shows the lines that create four instances of the Dog class: jake, duke, lady, and dakota. Then, the code creates a Party<Dog> instance named dogsParty and passes jake as the leader argument to the initializer. This way, we will create a party of dogs, and Jake is the party leader:

var jake = Dog(name: "Jake")
var duke = Dog(name: "Duke")
var lady = Dog(name: "Lady")
var dakota = Dog(name: "Dakota")
var dogsParty = Party<Dog>(leader: jake)

The dogsParty instance will only accept a Dog instance for...