空无课程是指学校中应该有而没有的课程，教科书的空无内容是指教科书中应该呈现而没有呈现的知识。
The null content of a textbook means such a kind of content that should be contained in the textbook but not.
空无课程是指学校中应该有而没有的课程，教科书的空无内容是指教科书中应该呈现而没有呈现的知识。
The null content of a textbook means such a kind of content that should be contained in the textbook but not.
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