Re: What is the principle?

From: Peter Kirk (
Date: Wed Mar 31 2004 - 19:28:19 EST

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    On 31/03/2004 15:32, Ernest Cline wrote:

    >>[Original Message]
    >>From: Peter Kirk <>
    >>Ernest, I support your general ideas here. But I am concerned about the
    >>implications of defining PUA characters with combining classes other
    >>than zero. I can see this causing some confusion with normalisation etc.
    >>And it does hugely multiply the number of PUA characters required.
    >>Is it really necessary to support to this level of detail the concept of
    >>canonical equivalence of PUA sequences?
    >If you want them to be able to interact with the existing combining marks
    >then any proposal for more specific private use characters will need to
    >include combining characters for every existing combining class. ...

    I don't see it. If the PUA combining marks have cc=0, they can never be
    reordered. As long as other marks are always written in canonical order,
    they will in practice never be moved relative to other marks.

    Perhaps you are thinking of a sequence something like <B, M1, M2, M3> in
    which M1 and M2 interact typographically, but M1 is PUA and M2 and M3
    are not. The normal Unicode rule would be that cc(M1)=cc(M2). But this
    is no guarantee against a reordering to <B, M1, M3, M2>, which is still
    canonically equivalent but M1 and M2 have been separated. If instead
    cc(M1)=0<cc(M2)<cc(M3), <B, M1, M3, M2> is still canonically equivalent
    but with M1 and M2 separated, but the situation is no worse than by the
    normal Unicode rule.

    Peter Kirk (personal) (work)

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