# File lib/mofo/xoxo.rb, line 24
  def self.build_tree(child)
    tree = []
    child.search(@@children_xpath) do |element|
      label, branch = nil, nil
      element.children.each do |inner|
        label  ||= build_label(inner) unless container?(inner)
        branch ||= build_tree(inner) if container?(inner)
      end
      tree << (branch ? { label => branch } : label)
    end 
    tree
  end