On Sun, 2 Feb 2020 07:51:56 -0800
Ken Whistler via Unicode <unicode@unicode.org> wrote:

> What it comes down to is avoidance of conundrums involving canonical 
> reordering for normalization. The effect of variation selectors is 
> defined in terms of an immediate adjacency. If you allowed variation 
> selectors to be defined for combining marks of ccc!=0, then 
> normalization of sequences could, in principle, move the two apart.
> That would make implementation of the intended rendering much more
> difficult.

I can understand that for non-starters.  However, a lot of non-spacing
combining marks are starters (i.e. ccc=0), so they would not be a
problem.  <starter, variation selector> is an unbreakable block in
canonical equivalence-preserving changes.  Is this restriction therefore
just a holdover from when canonical equivalence could be corrected?


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