ProtocolExtension.swift

protocol Refuelable {
    func refuel()
}

protocol Drivable {
    func drive()
}

struct Car: Refuelable, Drivable {}

//If we just include everything above this comment, the code will not compile.
//We need the default implementations below:

extension Refuelable {
    func refuel() {
        //Default refuel code here
    }
}

extension Drivable {
    func drive() {
        //Default drive code
        print("Default drive speed is 30mph")
    }
}

struct CustomCar: Refuelable, Drivable {
    
    func drive() { //Declaring this function within a struct that conforms to "Drivable" is equivalent to "override" for an inherited class type
        print("The CustomCar drives at 100mph")
    }
    
}

let car = Car()
car.drive() // Prints "Default drive speed is 30mph"

let customCar = CustomCar()
customCar.drive() // Prints "The CustomCar drives at 100mph