def next_sibling()
      if siblings = parent.children
        siblings.each_with_index do |node, i|
          return siblings[i + 1] if node.equal?(self)
        end
      end
      nil
    end
def next_sibling()
      if siblings = parent.children
        siblings.each_with_index do |node, i|
          return siblings[i + 1] if node.equal?(self)
        end
      end
      nil
    end
Returns the next sibling node.