code: def validate_milestones(self):
          '''
          Reviews the arc element tree to ensure that milestones appear in the right
          order.
          '''
          milestones = self.arc_root_node.get_children().filter(arc_element_type__contains='mile')
          current_cursor = 0
          for mile in milestones:
              seq = mile.milestone_seq
              if seq < current_cursor:
                  return mile
              current_cursor = seq
          return Nonf validate_milestones(self):
          '''
          Reviews the arc element tree to ensure that milestones appear in the right
          order.
          '''
          milestones = self.arc_root_node.get_children().filter(arc_element_type__contains='mile')
          current_cursor = 0
          for mile in milestones:
              seq = mile.milestone_seq
              if seq < current_cursor:
                  return mile
              current_cursor = seq
          return None
docs: Reviews the arc element tree to ensure that milestones appear in the right
          order.