class Tree
    attr_reader :value
    def initialize(value)
      @value = value
      @children = []
    end
 
    def <<(value)
      subtree = Tree.new(value)
      @children << subtree
      return subtree
    end
  end
 
# Here�s code to create a specificTree(Figure 7-1):
 
  t = Tree.new("Parent")
  child1 = t << "Child 1"
  child1 << "Grandchild 1.1"
  child1 << "Grandchild 1.2"
  child2 = t << "Child 2"
  child2 << "Grandchild 2.1"
 
class Tree
    def each
      yield value
      @children.each do |child_node|
        child_node.each { |e| yield e }
      end
    end
  end
 
# The each method traverses the tree in a way that looks right:
 
  t.each { |x| puts x }