extension_functions.kt

open class Vehicle {
//    open fun startEngine() = println("Vroom!")
}
open class Car : Vehicle() {
//    open override fun startEngine() = println("Vroom vroom!")
}
fun Vehicle.startEngine() {
    println("Vroom!")
}
fun Car.startEngine() {
    println("Vroom vroom!")
}

fun main() {
	  val v = Vehicle()
    val c = Car()
    
	  v.startEngine() // => "Vroom!"
	  c.startEngine() // => "Vroom vroom!"
}