class Tree(val x: Int, val left: Tree? = null, val right: Tree? = null)

fun order(T: Tree?) = sequence {
    if (T == null) return@sequence
    yieldAll(order(T.left))
    yield(T.x)
    yieldAll(order(T.right))
}

fun main() = println(order(Tree(2, Tree(1), Tree(3))).toList())