The example below is perfectly well-formed and valid, and it might make sense, in a network setting, to transmit the whole DTD in the internal subset as in this example.
However, it would almost never make sense to author or maintain the document in this form. Typically, a DTD is used to describe and control the authoring of many documents, not just one; for this reason it makes sense to store it separately and point at it from each of those documents.
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