While it is almost never used as far as I know, early on in METS development it was suggested that people might need to indicate sequences of files within a <par> element. This might be useful, for example, when you have parallel audio and video streams where both the audio and video streams are kept in multiple files. It's a slight stretch of the semantics that were originally intended for the <par> tag, but if you want to indicate that the order of <area> elements within a <par> element is significant, I'd probably code that as follows:
<fptr>
<par>
<seq>
<area FILEID="master.2" />
<area FILEID="master.3 />
<area FILEID="master.4 />
…
</seq>
</par>
</fptr>
You are allowed to put multiple <seq> elements within a single <par> element, so if you wanted to separate out different sequences within a <par>, you can do that as well. Admittedly this is not the most perfect of solutions, but I'm afraid 3D models were one of the use cases I went out of my way to ignore in METS' initial <structMap> design, so without some revisions to METS' design, handling of 3D materials will of necessity be a bit of a hack.
best,
Jerome McDonough
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