[XeTeX] VIQR pre-processor wrotten in (Xe)TeX ?

[Philip TAYLOR]

Bruno Le Floch wrote:
> Is it simply a matter of going through the string and replace various
> characters by TeX accents (and take care of character order), or does
> the result have to be Unicode?
>
> E.g., does it have to be a(a^ → ăâ, or can it be a(a^ → \u{a}\^{a} ?
> If the second one is ok, then it shouldn't be too hard to write the
> conversion in TeX macros. Please provide a list of the necessary
> conversion rules.

Vietnamese is difficult for TeX, because over a single
character there can be a requirement for both an
accent (to indicate a pronunciation different from
that for the unaccented letter) and a tone marker
(which applies to the whole word, but which has
a canonical placement that may well be on an
already accented letter).  Hàn Thế Thành created
a way of accomplishing this using standard TeX,
but XeTeX can do it natively.  Incidentally,
the "ế" at the end of Thành's middle name demonstrates
exactly the problem : something that TeX cannot
accomplish without special fonts, since it has
no primitive for positioning two diacritics over
a single character.

Philip Taylor