[luatex] Handling of combining and variant selector characters in math

[David Carlisle]

Sending this separately to xetex  luatex lists...

The current Unicode math alphabets in U+1D400 ...include a "script"
alphabet this was intended to cover both "caligraphic/chancery" and
"script/roundhand" as font variants, in the hope that no document
would need both.

Unicode is considering adding separate markup for the two forms, see
Murray's blog here

http://blogs.msdn.com/b/murrays/archive/2016/02/05/unicode-math-calligraphic-alphabet.aspx

As explained in the article at least two possible suggestions are
being considered:  adding the new alphabet in a new code block range,
or defining  "variant selector" characters that would force one or
other interpretation.

Unicode combining characters following the base have always been a bit
tricky in TeX so I wondered whether the engine (or font) developers
(as opposed to macro level hacker like myself) have a view on what is
a reasonable input form here.

You could either reply here or as a comment on the above Blog.

I attach a (latex) text file that produces more or less the same
output in xetex and luatex showing that by default neither a simple
combining character like e-acute nor VS1 work but the combining acute
at least can perhaps be made to work but VS1 seems tricky as the base
is a \mathop atom so it's not really amenable to being combined with
the following variant selector character.

Since current combining character use seems tricky I'm worried about
the suggestion to use that method for selecting the entire script
alphabet.

(The combining acute could be normalised away by running filter to NFC
form but don't do that as it's just standing in for a possible new
character to switch script forms:-)

Tests 0 and A show that both combining forms work fine in text, but
math is the issue here....

David
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