This function can be used to distinguish between the two instances in which the function doc_freeform returns 1 (True). If the document identified by doc is an XML instance without a DTD declaration, doc_dtd_not_defined returns 1 (True). If the document's declared DTD cannot be found, and the user has selected to edit in tagged mode without a DTD, this function will return 0.

If doc_freeform returns 0, doc_dtd_not_defined will also return 0.

The doc argument specifies the identifier of the document to query. If omitted or 0, the current document is used.