一个响应包含一个或多个命令消息。
A response contains one or more command messages. Some of the commonly used command messages are discussed in Table 1.
子类可以重写这个对象发送命令消息给特定的对象。
Subclasses may override this method to perform special dispatching of action messages.
这一协议主要包含两部分：命令消息和状态转换机制。
There are two main segments in this protocol: message and statement transition mechanism.
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