Vehicle2.swift

protocol Vehicle {
    var currentSpeed : Double { get }
    
    func accelerate()
    func decelerate()
}

extension Vehicle {
    var isMoving : Bool { get { return currentSpeed != 0 } }
}

protocol GroundVehicle: Vehicle {
    var numberOfWheels : Int { get }
}

protocol AirVehicle: Vehicle {
    func takeOff()
    func land()
}

protocol WaterVehicle: Vehicle {
    func submerge()
}