tree

 case class Tree[T](value: T, children: List[Tree[T]])

  /**
   *
   * @param tree
   * @param f A function to be applied to every node in the tree
   * @tparam S Output type
   * @tparam T Value type of node
   * @return A Queue[S] where the first element is the output of first node visited
   *
   *               1
   *              / \
   *             4   2
   *            /   / \
   *           3   8   5
   *          / \     / \
   *         10  9   6   7
   *
   */


  def include[S, T](value: T, acc: (Queue[S], Set[T]), f: T => S) = {
    val (out, vis) = acc
    if (!vis(value))
      (out.enqueue(f(value)), vis + value)
    else
      (out, vis)
  }