动态改变方法体，领略ruby强大的动态编程能力

ruby 代码

class A
  def meth
    pust "old method involked!"
    s = <<-EOF
      def meth
        puts "new method involked!"
      end
    EOF
    self.instance_eval(s)
  end
end

a = A.new
a.meth =>"old method involded"
a.meth =>"new method involded"

多态原来也可以这样做