Example tree implementation

node.rb

require 'pp'

Node = Struct.new :value, :parent, :children do
  def initialize(value = nil, parent = nil)
    self.value = value
    self.children = []
    yield self if block_given?
  end

  def add(child)
    children << child
    child.parent = self
  end

  alias << add

  def each_child(&b)
    b[self]
    children.each do |ch|
      ch.each_child(&b)
    end
    self
  end

  def root
    n = self
    n = n.parent while n.parent
    n
  end

  def doc_each_child(&b)
    root.each_child(&b)
    self
  end
end

foo = nil

tree = Node.new(1) {|x1|
  x1 << Node.new(2) {|x2|
    x2 << Node.new(3) << Node.new(4)
    foo = Node.new(5)
    x2 << foo
  } << Node.new(6)
}

# XPath '*'
foo.each_child do |ch|
  printf "Node %5s\n", ch.value
end

puts '-----------------'

# XPath '//*'
foo.doc_each_child do |ch|
  printf "Node %5s\n", ch.value
end