trait Hierarchical[T <: Hierarchical[T]] {
  val parents: List[Hierarchical[_]] = Nil
  var children: List[Hierarchical[T]] = List()
}
 
class NonRootNode(override val parents: List[Hierarchical[_]]) 
  extends Hierarchical[NonRootNode] {}
 
class Root extends Hierarchical[NonRootNode] {
  final override val parents = Nil
}
 
object Main extends App {
  val root = new Root
  assert(root.parents.isEmpty)
  val child = new NonRootNode(List(root))
  assert(child.parents.size == 1)
  assert(child.parents.contains(root))
  println("Success...")
}

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