Sets: Firefox 3.0 implements some useful extensions for working with node sets.
Sets： Firefox 3.0实现了一些使用节点集的有用扩展。
This (in the case of expressions representing node sets) allows the nodes to be retrieved incrementally, rather than all at once.
这（在表达式表示节点集的情况下）允许递增检索节点，而不是立即检索所有节点。
So long as the operands are both already sorted into document order, this avoids the need to allocate memory for the intermediate node sets.
只要已经按文档顺序排序了这两个操作数，就不必为中间节点集分配内存。
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