new_contents: from traitlets import Bool, HasTraits, Instance, Set, This
  from .tree import Tree
  
  
  class HasTree(HasTraits):
      _tree = Instance(klass=Tree, allow_none=True)
      _owner = This(allow_none=True)
  
      def __init__(self, tree=None, **kwargs):
          super().__init__(**kwargs)
          self.tree = tree
  
      @property
      def tree(self):
          return self._tree
  
      @tree.setter
      def tree(self, src):
          self.set(src)
  
      @property
      def owner(self):
          return self._owner
  
      def set(self, src):
          if self._owner is not None:
              self._owner.unobserve(self.ref_tree_changed, names='_tree')
          self.update(src)
  
      def ref_tree_changed(self, change):
          self.update(self._owner)
  
      def update(self, src):
          if isinstance(src, Tree):
              self._owner = None
              self._tree = src
          elif isinstance(src, HasTree):
              self._owner = src
              self._tree = src.tree
              self._owner.observe(self.ref_tree_changed, names='_tree')
subject: move to the tree module