grand_parent1
  parent1 (grand_parent1)
    parent1-child1 (parent1 << grand_parent1)
    parent1-child2 (parent1 << grand_parent1)
  parent2 (grand_parent1)
    parent2-child1 (parent2 << grand_parent1)
    parent2-child2 (parent2 << grand_parent1)
