I'm surfacing an issue from [EMAIL PROTECTED] because it may have wider applicability.
Currently, it's the rule that variation selector characters can't be applied to combining characters. This is sensible in the case of true diacritical marks: if two marks differ in shape, they ought in general to be encoded separately, since marks are primarily shape-based rather than functional in the first place.
It's not so clear, however, that this rule is appropriate when applied to vowel signs. If some textual traditions represent vowels a and a' differently whereas others unify them, and if the variation is not predictable at the Unicode level, then it would seem appropriate to provide a vowel sign for a and define a VSS <a, VS1> to represent a' in those textual traditions in which it is in use. The alternative of providing a distinct vowel sign a' and treating the difference as one of spelling impacts backward compatibility and burdens textual processes.
The rationale for making a variation selector ineligible to apply to combining marks comes from normalization. In normalization all combining character sequences are put in their canonical order, based on their combining class. If we want to allow accents to be placed on base characters to which a variation selector has been applied, then the variation selector needs to come in between the base character and the accent. Unless we make it a combining character, the VS would interrupt the combining character sequence (separating the accent from the base character). But if it is a a combining character, it takes part in the reordering. Therefore, it needs to have combining class 0, so it stays with the base character. And that's the reason it cannot be used to apply to characters whose combining class is not 0.
So you see, the rule is not based on the linguistic nature of the combining character, but on its combining class.
A./
http://www.unicode.org/reports/tr15/ (Normalization) http://www.unicoce.org/Public/UNIDATA/extracted/DerivedCombiningClass.txt
