内容模型必须是确定性的。对元素“<var>X</var>”进行多次定义会导致内容模型变得不确定。

Multiple definition of element '<var>X</var>' causes the content model to become ambiguous. A content model must be formed such that during validation of an element information item sequence, the particle contained directly, indirectly or implicitly therein with which to attempt to validate each item in the sequence in turn can be uniquely determined without examining the content or attributes of that item, and without any information about the items in the remainder of the sequence.

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