code: def _observe_block(self, change):
          
          if self.is_initialized:
              if change['type'] == 'update':
                  old_block = change['oldvalue']
                  old_block.parent.remove_children(old_block,self.children)
                  new_block = change['value']
                  new_block.parent.insert_children(new_block, self.children)
docs: A change handler for the 'objects' list of the Include.
  
          If the object is initialized objects which are removed will be
          unparented and objects which are added will be reparented. Old
          objects will be destroyed if the 'destroy_old' flag is True.