Normalization Generics (NFx, NFKx, NFxy)

[Richard Wordingham]

On Sun, 13 Dec 2020 20:08:08 -0800
Zach Lym via Unicode <unicode at unicode.org> wrote:

> > What does that quoted statement mean?  I'm having a hard job working
> > out what the meaning of full case folding is.  I'm not having any
> > doubts about the meaning of toCasefold(NFD(X)), so there is no issue
> > for 'canonical caseless matching'.  
> 
> The "case folding is closed under canonical normalization" or the
> other part?

That part.

> Closed as in closure:
> https://en.wikipedia.org/wiki/Closure_(mathematics)

That only tells me what it means for a _set_ to be closed under an
operation.  What does it mean for a _function_ (or similar) to be
closed under an operation?

If I must use the definition for a set, then I can only conclude that
for one operation to be closed under another operation, the result
should be independent of the order in which they are applied.

But for X = <U+1FB3 GREEK SMALL LETTER ALPHA WITH YPOGEGRAMMENI, U+0342
COMBINING GREEK PERISPOMENI>:

NFD(toCasefold(X)) = <U+03B1 GREEK SMALL LETTER ALPHA, U+03B9 GREEK
SMALL LETTER IOTA, U+0342>

toCasefold(NFD(X)) = <U+03B1, U+0342, U+03B9>

NFC(toCasefold(X)) = <U+03B1, U+1FD6 GREEK SMALL LETTER IOTA WITH
PERISPOMENI>

toCasefold(NFC(X)) = <U+1FB6 GREEK SMALL LETTER ALPHA WITH PERISPOMENI,
U+03B9>

So either "case folding is closed under canonical normalization" means
something else, or it is simply not true.

> Refer to page 240 of the standard, Chaper 5 "Implementation
> Guidelines" Section 18 "Case Mappings":
> 
> http://www.unicode.org/versions/latest/ch05.pdf

Why?

The trick is not to be deflecting by the opening paragraph in TUS
Section 3.13, but to read on to find R4.

Richard.