每个业务流程都处理一些易变数据，这些数据可用于支持流中的决策，或者用作某些流活动的输入或输出参数。

Every business process deals with some amount of variable data, that might be used for decisions within the flow or as input or output parameters of some flow activities.

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此函式用在设定变数的预设值，如果变数的值是空白或尚未设定的话，则会列印出预设值，使用此函式时会预设需要一个参数。

This is used to set a default value for a variable. If the variable is empty or unset, the given default value is printed instead. Default takes one argument.

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