open class Animal
 
class Cat() : Animal() {
    fun myaw() {
        System.out.println("myaw!");
    }
}
 
class Dog() : Animal() {
    fun gav() {
        System.out.println("dog!");
    }
}

// Usage:
//     val list = mutableListOf<Int>()
//     list.addAnything("string")
//     val fail = list[0]
private fun <E> List <E>.addAnything(element: E) {
	if (this is MutableList<E>) {
		this.add(element)
	}
}

fun main() {
    val cats = mutableListOf<Cat>(Cat(), Cat())
    val upcast: List<Cat> = cats
    val covariance: List<Animal> = upcast
	val animals: MutableList<Animal> = covariance as MutableList
    animals[0] = Dog()
    for (cat in cats) {
        cat.myaw()
    }
}