tree.scala

abstract class Tree
case class Leaf(value: String) extends Tree
case class Branch(left: Tree, right: Tree) extends Tree

def walk(t: Tree): Unit =
  t match {
		case Branch(Stub, _) => println("branch-with-stub-on-the-left")
		case Branch(left, right) => {
			println("\nbranch-leaf")
			walk(left)
			walk(right)
		}
		case Leaf(v) => printf("\tleaf: %s\t", v)
	}

walk(Branch(
	Leaf("A"), Branch(
		Branch(Leaf("D"),Leaf("B")), Leaf("C"))))