They are, but this has never been documented and so took me a long time to discover (literally years!) See:
I have not taken this as final though, because:
a) without being documented it is not reliable IMO.
b) it is unsatisfactory (again IMO) and therefore should be subject to discussion before being finalised, but we can’t discuss something that isn’t documented.
c) the implementation or how best to use it might be affected by changes introduced for Appendable Data.
Hopefully with the roadmap in place we’ll get an idea of when this might be discussed or finalised.