Returns list of ancestors, starting from parent until root.
subchild1.ancestors # => [child1, root]
Returns the root node of the tree.
Returns all siblings and a reference to the current node.
subchild1.self_and_siblings # => [subchild1, subchild2]
Returns all siblings of the current node.
subchild1.siblings # => [subchild2]
[Validate]