package test

trait Ordered[a] {
  def < (x: a): Boolean
}

object O {
  implicit def view (x: String): Ordered[String] = new Ordered[String] {
    def < (y: String) = x.compareTo(y) < 0
  }
}

object Empty extends Tree[Nothing]
case class Node[c <% Ordered[c]](elem: c, l: Tree[c], r: Tree[c]) extends Tree[c]

abstract class Tree[+a <% Ordered[a]] {
  def insert[b >: a <% Ordered[b]](x: b): Tree[b] = this match {
    case Empty =>
      new Node(x, Empty, Empty)
    case Node(elem, l, r) =>
      if (x == elem) this
      else if (x < elem) Node(elem, l insert x, r)
      else Node(elem, l, r insert x)
  }
  def elements: List[a] = this match {
    case Empty => List()
    case Node(elem, l, r) =>
      l.elements ::: List(elem) ::: r.elements
  }
}

object Test {
  import O.view

  def main(args: Array[String]): Unit = {
    var t: Tree[String] = Empty
    for (s <- args) {
      t = t insert s
    }
    println(t.elements)
  }
}