Only if the WHEN clause evaluates to true will the action part of trigger statement be executed.

仅当when子句的计算结果为真时，触发器语句的操作部分才会执行。

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The while loop, as was the case with the if statement, supports an optional else clause containing a block of program statements executed when the expression is False.

while循环与if语句一样，支持一个可选的else子句，其中包含一个当表达式计算为False时执行的程序语句块。

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If the match is successful, the message is received from the mailbox, the message arguments are made available by binding them to the variables in the match, and the corresponding clause is executed.

如果匹配成功，则从邮箱接收消息，消息参数通过绑定到匹配中的变量提供，并执行相应的子句。

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