[Cellar] Adding private elements to Matroska

[Jerome Martinez <[e-mail]>]

I am working on a project related to Matroska and I would like to add 
some private elements to Matroska, at different levels (TrackEntry, 
BlockGroup, Segment).
These elements add no specific features for media playback.

As there is no indication about how to add private elements (no element 
is reserved for hosting private sub elements), Matroska conformance 
checkers currently flag new elements as "unknown".

I would like to debate about how to add new elements to Matroska, for 
some specific needs so more flagged "private", and for future needs for 
Matroska (e.g. HDR metadata was added during CELLAR work, what should be 
done in the case CELLAR already released the spec?).

Some questions:
- IMO a new element should not make a new version mandatory, as new 
element are ignored by players, but what should be the behavior of a 
conformance checker? error raising if the element is not known, 
information only? Maybe we need a paragraph about it in specs.
- How to manage private elements? Difficulty is to be sure an element 
identifier used by third party A is not reused by third party B due to B 
not knowing about A.On my side I suggest to just add it to 
ebml_matroska.xml without specific details in spec itself, see 
https://github.com/Matroska-Org/matroska-specification/pull/223/files 
for an example, so everyone is aware of identifiers reserved for a third 
party tool. Should such elements be in a separate document, mp4ra style?

Jérôme