As an example, Listing 2 can leverage the existence or non-existence of positional parameters to provide a helpful usage message if you specify no arguments.
例如，如果您未指定任何参数，则清单2可以利用位置参数的存在性或非存在性来提供有帮助的用法信息。
2
In Tristan and Isolde, this release from life is achieved by the self-sacrificial love of a woman who is prepared to share her lover’s non-existence and unite with him in death.