object Variance
{
  // to allow 
  trait List[+T] {
    def isEmpty: Boolean
    def head: T
    def tail: List[T]
  }

  class Cons[T](val head: T, val tail: List[T]) extends List[T] {
    def isEmpty = false
  }

  // extends List[Nothing] so that it can hold anything
  object Nil extends List[Nothing] {
    def isEmpty = true
    def head: Nothing = throw new NoSuchElementException("Nil.head")
    def tail: Nothing = throw new NoSuchElementException("Nil.tail")
  }

  def run = {
    println("Variance")
    val x: List[String] = Nil
  }
}