Here the head is a single element, and the tail is the rest of the list.
头是单一元素，尾是列表的其余部分。
This call will remove the year element only; the rest of the document remains unchanged.
这次调用将只除去year元素；文档的其余部分保持不变。
This call will remove the year element only; the rest of the document remains unchanged.
这次调用将只除去year元素；文档的其余部分保持不变。
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