color = :red 和 self.color = :orange，为什么第一个是本地变量赋值，而第二个是方法调用？为什么第一个不是隐含的 对 self 调用 color=() 方法呢？

class TrafficLight
  attr_accessor :color

  def progress_color
    case color
    when :orange
      #Don't do this!
      color = :red
    when :green
      #Do this instead!
      self.color = :orange
    else
      raise NotImplementedError, "What should be done if color is already :red? Check with the domain expert, and build a unit test"
    end
  end
end

traffic_light = TrafficLight.new
traffic_light.color = :green
traffic_light.progress_color
traffic_light.color # Now orange
traffic_light.progress_color
traffic_light.color # Still orangea