case class Inner(left: Tree, right: Tree) extends Tree
  case class Leaf(i: Int) extends Tree

    abstract class Generator[+T] {
    self =>

    def generate: T

    def map[S](f: T => S): Generator[S] = new Generator[S] {
      def generate = f(self.generate)
    }

    def flatMap[S](f: T => Generator[S]): Generator[S] = new Generator[S] {
      def generate = f(self.generate).generate
    }
  }

  val integers = new Generator[Int] {
    val rand = new Random
    def generate: Int = rand.nextInt()

  }

  val booleans: Generator[Boolean] = for{
    x <- integers
  } yield x > 0


  val trees: Generator[Tree] = for{
    isLeaf <- booleans
    tree <- if(isLeaf) leaves else inners
  } yield tree

  val leaves: Generator[Leaf] = for{
    i <- integers
  } yield Leaf(i)

  val inners: Generator[Inner] = for{
    i <- trees
    j <- trees
  } yield Inner(i, j)