Tree Algebraic Data Structure with different traverse orders implemented in Kotlin

TreeADT.kt

import java.util.*

inline fun <T, R> T.ifNotNull(callback: T.() -> R): R? {
    return this?.callback()
}

sealed class Tree<T> {

    abstract val value: T;

}

class Node<T>(val left: Tree<T>?, val right: Tree<T>?, override val value: T) : Tree<T>() {


}

class Leaf<T>(override val value: T) : Tree<T>() {

}

enum class TraverseOrder {

    PRE {
        override fun <T> visitNode(t: Node<T>, body: T.() -> Unit) {
            t.value.body()
            traverse(t.left, body)
            traverse(t.right, body)
        }
    },
    POST {
        override fun <T> visitNode(t: Node<T>, body: T.() -> Unit) {
            traverse(t.left, body)
            traverse(t.right, body)
            t.value.body()
        }
    },
    IN {
        override fun <T> visitNode(t: Node<T>, body: T.() -> Unit) {
            traverse(t.left, body)
            t.value.body()
            traverse(t.right, body)
        }
    },
    LEVEL {
        override fun <T> visitNode(t: Node<T>, body: T.() -> Unit) {
            throw NotImplementedError()
        }

        override fun <T> traverse(t: Tree<T>?, body: T.() -> Unit) {
            val queue: Queue<Tree<T>> = LinkedList(listOf(t))
            while (queue.isNotEmpty()) {
                val v = queue.poll()
                v.value.body()
                if (v is Node) {
                    v.left.ifNotNull(queue::add)
                    v.right.ifNotNull(queue::add)
                }
            }
        }
    };

    protected abstract fun <T> visitNode(t: Node<T>, body: T.() -> Unit);

    open fun <T> traverse(t: Tree<T>?, body: T.() -> Unit) {
        when (t) {
            is Node -> visitNode(t, body)
            is Leaf -> t.value.body()
        }
    }

}

fun main() {

    val tree: Tree<Int> = Node(
        Node(
            Leaf(5),
            null,
            1
        ),
        Node(
            Leaf(3),
            Leaf(4),
            2
        ),
        0
    )

    println("IN")
    TraverseOrder.IN.traverse(tree) {
        println(this)
    }
    println()
    println("PRE")
    TraverseOrder.PRE.traverse(tree) {
        println(this)
    }
    println()
    println("POST")
    TraverseOrder.POST.traverse(tree) {
        println(this)
    }
    println()
    println("LEVEL")
    TraverseOrder.LEVEL.traverse(tree) {
        println(this)
    }

}