``` open class A class B : A() open class X { ...

# getting-started

[jkbbwr]

open class A
class B : A()

open class X {
    open val childen: List<A> = emptyList()
}

class Y : X() {
    override val childen: List<B> = emptyList()
}

fun main(args: Array<String>) {
    Y()
}