Nemerel tree converted to Scala

TreeMagic.scala

package problems

trait Comparable[T] {
  def compareTo(t: T): Int
}

case class ComparableInt(val a: Int) extends Comparable[ComparableInt] {
  override def compareTo(b: ComparableInt): Int = {
    if(a < b.a) {
      -1
    } else if (a > b.a) {
      1
    } else {
      0
    }
  }
}

abstract class Tree[A <: Comparable[A]]

case class Node[A <: Comparable[A]](l: Tree[A], c: A, r: Tree[A]) extends Tree[A] 
case class Tip[A <: Comparable[A]] extends Tree[A]

object Tree {
  def insert[A <: Comparable[A]](t: Tree[A], e: A): Tree[A] = {
    t match {
      case Node(l, c, r) => {
        e.compareTo(c) match {
          case -1 => Node(Tree.insert(l, e), c, r)
          case 1 => Node(l, c, insert(r, e))
          case 0 => Node(l, e, r)
        }          
      }
      case Tip() => Node(Tip(), e, Tip())
    }
  }
}

object TreeMagic extends App {
  val a = ComparableInt(123) 
  val b = Node(Tip(), a, Tip())
  val c = Tree.insert(b, ComparableInt(100))
  
  println(c)
}