A gate is merely a message that is illustrated with one end connected to the sequence diagram's frame's edge and the other end connected to a lifeline.
一个门只是一个消息，图形表示为一端连接序列图的框架边缘，另一端连接到生命线。
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Summary: From The Rational Edge series of articles, .UML basics, on the essential diagrams in the Unified Modeling Language, this article offers a detailed introduction to the sequence diagram.