class Nokogiri::XML::Node

def ancestors selector = nil

the ancestors must match +selector+
Get a list of ancestor Node for this Node. If +selector+ is given,
## 
def ancestors selector = nil
  return NodeSet.new(document) unless respond_to?(:parent)
  return NodeSet.new(document) unless parent
  parents = [parent]
  while parents.last.respond_to?(:parent)
    break unless ctx_parent = parents.last.parent
    parents << ctx_parent
  end
  return NodeSet.new(document, parents) unless selector
  root = parents.last
  NodeSet.new(document, parents.find_all { |parent|
    root.search(selector).include?(parent)
  })
end