The Core Group has agreed to encourage the use of ExtPos to specify how a word (or the expression/phrase it heads) functions with respect to its external deprel, where this may differ from the UPOS reflecting the word's morphology and dependents.

In particular, the first word of every fixed expression should specify ExtPos to reflect the UPOS that the whole expression would bear were it a single word.

Some treebanks have already been using ExtPos in MISC, others in FEATS. @dan-zeman has said that MISC is the appropriate place as it reflects properties broader than a single word's morphology.

I assume we need to the following:

1. A description of how ExtPos should be used (at least for fixed, though treebanks may opt to use it for art titles, SYM, and so on). Where will this live? The MISC attributes page is a bit cluttered with treebank-specific/experimental attributes. Cf. https://universaldependencies.org/en/feat/ExtPos.html
2. This should be mentioned in the fixed guidelines.
3. An update to the validator that checks ExtPos if present and uses that rather than UPOS for checks on the incoming deprel.
4. An update to the validator that warns if ExtPos is missing for a fixed expression. (not an error)
5. An announcement blurb for the changes page.