做七周七语言ruby第二天习题的时候要实现一个简单的Tree类，以下代码可以运行，但是把
children.each {|c| c.visit_all(n+1) {|node| puts "-#{node.node_name}"}}
这行的{|node| puts "-#{node.node_name}"}改成&block就跑不起来，请问是为什么？

#!/usr/bin/ruby
class Tree
  attr_accessor :children,:node_name

  def initialize(tree)
    tree.each do |key,value|
      @node_name = key
      @children = value.map {|(key,value)| Tree.new(key => value)}
    end
  end

  def visit_all(n,&block)
    visit &block
    print '  ' * n
    children.each {|c| c.visit_all(n+1) {|node| puts "-#{node.node_name}"}}
  end

  def visit(&block)
    block.call self
  end

end

ruby_tree = Tree.new({
    'grandpa' => {
        'day' => {'child 1' => {},'child 2' => {}},'uncle' => {'child 3' => {},'child 4' => {}}
    }
})

ruby_tree.visit_all(1) {|node| puts "-#{node.node_name}"}

ps: ruby环境是2.1.3