Since you're looking at the NCX code, according to the
epub 2.0.1. specs:
Quote:
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The NCX file must be a valid XML document conforming to ncx-2005-1.dtd, with the exception of the playOrder attribute, which is optional in EPUB NCX
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and
Quote:
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Any NCX that contains a DOCTYPE that references the canonical NCX DTD must honor that DTD, thus including the playOrder attribute in all locations where it is required. NCX documents that do not contain a DOCTYPE may omit the playOrder attribute.
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[Emphasis mine]
I.e., if you didn't include the ncx-2005-1.dtd DOCTYPE declaration, you could also remove the playOrder attributes, which should theoretically simplify NCX updates.