UML 1 . x中，为了在一个序列图上画一个约束，你把约束元件放在约束的消息线上，消息名字之前。

To draw a guard on a sequence diagram in UML 1.x, you placed the guard element above the message line being guarded and in front of the message name.

youdao

UML 2已经通过去掉“in - line”约束，增加一个叫做组合碎片的符号元件，解决了这一个问题。

UML 2 has addressed this problem by removing the "in-line" guard and adding a notation element called a Combined Fragment.

youdao

例子图 9的序列图片段包括一个选择项约束；然而，约束不是一个必需的元件。

The example Figure 9 sequence diagram fragment includes a guard for the option; however, the guard is not a required element.

youdao