object Test {
 sealed abstract class Tree[+T] {
  def leafCount: Int
 }

 case class Node[+T](value: T, left: Tree[T], right: Tree[T]) extends Tree[T] {
  def leafCount: Int = (left, right) match {
    case (End, End) => 1
    case _          => left.leafCount + right.leafCount
  }
 }

 case object End extends Tree[Nothing] {
  def leafCount: Int = 0
 }

 object Node {
    def apply[T](value: T): Node[T] = Node(value, End, End)
  }

 def main(args:Array[String]) = {
   println(Node('x', Node('x'), End).leafCount)
}