The nextfocus and prevfocus attributes contain IDREFs of other elements.
nextfocus 和 prevfocus 属性包含其他元素的 IDREF。
They are used about as much as XML's ID and IDREFs, which is to say that some applications will use them a lot, and some never.
其应用情况与XML的ID和IDREF相似，也就是说，有些应用程序会大量使用，而有些应用程序则根本不使用它们。
In other words, given an IDREF or IDREFS field, it's possible to quickly find the element or elements with the associated ID or IDs, but not the other way around.
换句话说，给定一个IDREF或idrefs域，可能会迅速找到有一个或多个相关标识的一个或多个元素，反之则不行。
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