docs: def _remove_tree(self, tree, parent=None):
          """
          Really remove the tree identified by `tree` instance from all indexes
          from database.
  
          Args:
              tree (obj): :class:`.Tree` instance.
              parent (obj, default None): Reference to parent.
          """
          # remove sub-trees
          for sub_tree in tree.sub_trees:
              self._remove_tree(sub_tree, parent=tree)
  
          # remove itself
          for index in tree.indexes:
              if not getattr(tree, index):
                  continue
  
              self._remove_from(
                  getattr(self, index + "_db"),
                  getattr(tree, index),
                  tree,
              )
  
          if parent:
              self._remove_from(self.parent_db, tree.path, parent)
  
          self.zeo.pack()