Adding Lowercase Letters (was: Uppercase is coming? (U+1E9E))

From: Richard Wordingham (
Date: Mon May 07 2007 - 23:21:50 CDT

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    Kenneth Whistler wrote on Tuesday, May 08, 2007 at 3:00 AM
    Subject: Re: Uppercase is coming? (U+1E9E)

    > I assume you are talking about the discussions of casefolding
    > stability, which now specify that if there is an existing
    > *uppercase* letter in the standard but no lowercase for it,
    > that a lowercase paired letter cannot be added later, as
    > casefolding stability would prevent adding a tolowercase() mapping
    > for it, and failing that, the expectations about the case
    > relation would not be met.

    Where is this specified?

    The relevant rules seem to be that:

    (A) toCasefold(NFKC(S)) is constant over time once all characters in S are
    (B) From each [equivalence] class [defined for case folding], one
    representative element (a single lowercase letter where possible) is chosen
    to be the common form. (TUS 5.0 p188 R5)

    I am not sure if the definition in TUS of case folding is mandatory -
    dotless 'i' and dotted 'I' are described as 'an exception', as though there
    might be other exceptions.

    However, if condition (A) is taken as influencing the application of
    condition (B), then as I read it, one could add an uppercase letter and then
    its lowercase form in a subsequent version, but they would then have to
    casefold to the uppercase letter. That seems better than not meeting the
    expectation that Unicode will eventually support the normal writing system
    of one's community.

    I can't deduce the prohibition on changing tolowercase().


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