A参考集从中删除r-stack和destroyed as soon as its originating mapping rule is deactivated. If there is a need to keep裁判erencesfor a longer period then its参考集can be made to persist on the sets that still reside in ther-stack。A参考集whose originating mapping rule is being deactivated can be made to persist either at the top or at the bottom of ther-stack。
这persistattribute in the<参考文献>element allows a参考集to be persisted beyond the deactivation of its originating mapping rule. The valid values for thepersistattribute are真的和false。A参考集默认情况下不是持久的，所以缺席persistattribute has the same effect as setting it tofalse。
<参考文献persist=”true|false”>
。。。
默认情况下，持久性参考集persists in the set at the top of ther-stack。This persistent set will be destroyed as soon as its containing set is also destroyed. For convenience, the xfMap allows a set to be persisted in the set at the bottom of ther-stack。这<参考文献>元素的可选persist-inattribute can be used to control this. The valid values for thepersist-inattribute are父和base。Setting thepersist-inattribute tobaseallows a set to be persisted at the bottom of ther-stack。默认情况下，持久性参考集将在堆栈顶部的集合中坚持 - 这与设置相同persist-inattribute to父。
<参考文献persist=”true”persist-in=”parent|base”>
。。。
A reference-set坚持另一个参考集overwrites the references in the host set.
Persist options may also be set with xfMap expression elements via children elements. The value for the name attribute may be persist or persist-in.
例如：
<参考文献> ... expression or expression sequence evaluating to “true” or “false” ... ... expression or expression sequence evaluating to “parent” or “base” ... 。。。