如前面所述，表达式节点还支持简单的算术和逻辑表达式。
As mentioned, expression nodes also support simple arithmetic and logical expressions.
谓词是用于已有节点集的部分表达式。
A predicate is a partial expression that can be applied to an existing node set.
在上面列出的操作中，element1 也可以是运算结果为一系列节点的表达式。
In the options listed above, element1 could also be an expression that evaluates to a sequence of nodes.
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