SelfTypes.kt

import java.util.stream.Stream

abstract class NodeScaffold<THIS : NodeScaffold<THIS>> {

    private val children: List<THIS>? = null

    fun children(): Stream<THIS> {
        return children!!.stream()
    }

    fun grandChildren(): Stream<THIS> {
        return children!!.stream()
            .flatMap { child -> child.children() }
    }

}

abstract class SpecialNodeScaffold<THIS : SpecialNodeScaffold<THIS>> : NodeScaffold<THIS>()// special methods

abstract class VerySpecialNodeScaffold<THIS : VerySpecialNodeScaffold<THIS>> :
    SpecialNodeScaffold<THIS>()// more special methods

class Node : NodeScaffold<Node>()

class SpecialNode : SpecialNodeScaffold<SpecialNode>()

class VerySpecialNode : VerySpecialNodeScaffold<VerySpecialNode>()