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The rule of thumb is that you cannot use a single instance of a particular
heading level except for the first level. You must recast the heading
levels. In your example, I would make 2.1 part of 2.0 and change 2.1.1 and
2.1.2 to 2.1 and 2.2, respectively. Alternatively, you can create a filler
heading 2.1 or 2.2, as appropriate; e.g.,