fun main() {
    val tree = Node(
        1,
        listOf(
            Node(
                2, listOf(
                    Node(3),
                    Node(4),
                    Node(5)
                )
            ),
            Node(6)
        )
    )

    val list = preorder({ x -> print("$x ") }, tree) //.map { it.rootLabel }
    println()
    println(list)
}

fun preorder(transform: (Int) -> Unit, node: Node) : List<Node>{
    val els: () -> List<Node> = { node.subForest.flatMap { preorder(transform, it) }}
    transform(node.rootLabel)
    return listOf(node) + els.invoke()
}

data class Node(
    val rootLabel: Int,
    val subForest: List<Node> = emptyList()
)