From: Philippe Verdy (verdy_p@wanadoo.fr)
Date: Sun Jun 11 2006 - 06:34:42 CDT
From: "Richard Wordingham" <richard.wordingham@ntlworld.com>
> C: Unachievable Target:
> If X and Y are canonically equivalent, so are f(X) and f(Y). This can fail
> because one of the casing operations does not preserve canonical
> equivalence.
Thne this looks like a bug in this particular casing operation. Any casing function should preserve canonical equivalences. If not, it's because it's incorrectly implemented. For example a casing operation in YPOGEGRAMENI that transfoms it from a combining character into a non-combining iota letter in some casing but keeps it in another casing is unsafe or incorrectly implemented or insufficiently specified (that's where I would suggest updating the rule to require that the YPOGEGRAMANI (if encoded separately) should behave as if it was combined if the previous character, meaning that it may need to be reordered before some other diacritics, prior to applying the case mapping.
As a consequence, if Y1=uc(X1) and Y2=uc(X2) then it is not guaranteed that Y1+Y2=uc(X1)+uc(X2) will be canonically equivalent to uc(X1+X2). But the concatenation operation is already known for not preserving the canonical equivalences, notably when it is used on defective operands.
Applying a case mapping on an isolated YPOGEGRAMENI looks like a pathological (unsafe) case, because it's a defective combining sequence.
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