Comments on the Unicode Codepoints and IDNA Internet-Draft

[Frank Ellermann]

Stéphane Bortzmeyer wrote:

>> There is a reason why many code points such as "mathematical
>> fraktur capital B" or "black-letter capital C" are disallowed.
 
> Do you mind if I ask the reason? 

Of course I don't mind.  I'm also very interested in "rationale"
and "alternatives" questions.  Several points in the IDNA design
are *very* far from obvious.  It's certainly not about the goal
"get some LDH output for non-ASCII Unicode input".

In that specific case I'd need straight forward tool showing me
all properties of any given code point in a human readable form,
but I don't have that, and I'm too lazy to write it.

I'd guess that those two code points are not really letters in
an Unicode sense, and back in IDNA2003 folks decided to emulate
the spirit of "ASCII LDH" also for U-labels.  I cannot say WHY.

But taking this as given "no letter and no digit => disallowed"
is clear.  For the reasons why an obvious letter is no letter
in Unicode I better don't speculate.  I certainly don't need a
code point for "black-letter capital C", but admittedly I'm no
typesetter for mathematical books.
 
 Frank