策略表达式既允许使用简单的声明性断言，也允许使用更为复杂的条件断言。
Policy expressions allow for both simple declarative assertions as well as more sophisticated conditional assertions.
很简单，indexer让用户定位文档时，很难甚至无法以规则表达式来指定搜索条件，并且快速的执行。
Quite simply, indexer allows one to locate documents using search criteria that may be difficult or impossible to specify as a regular expression, and to do so quickly.
像条件语句一样，它允许一个简单的语句位于分支的表达式之前。
Like conditionals, they allow a simple statement to proceed the expression that is being switched upon.
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