[XeTeX] Looking for a better font selection method

[Jjgod Jiang]

Hi,

Since I'm a Chinese, I typeset Chinese documents for most of the
time, but actually, these ``Chinese documents'' are mixed with
Chinese characters and Western (mostly Latin) characters. So, for
example (in UTF-8):

    XeTeX ÅÅ°æϵͳµÄ Linux °æ±¾ÏÖÔÚ¿ÉÒÔ²âÊÔÁË£¬ËüÌṩÁËÓë Mac OS X
    ϵͳÏÂÒ»Ö嵀 Unicode ºÍ OpenType Ö§³Ö (µ«Ã»ÓÐ AAT Ö§³Ö)¡£

For these documents, using predefined command like \cjk{}
(in CJKsamples.tex) will be unacceptable, since too many \cjk{}
will appears, and vice versa (if we \setromanfont{some CJK font}
and define \noncjk{}).

So, a fallback mechanism will be appreciated or else we could
use some combined font (I noticed that a few month ago, some
one proposed a virtual font solution) to specify CJK font and
Western font seperately.

I know this will affect the line-breaking features and may not be
quite easy to achive, but could you please give me some workaround
(maybe in LaTeX level)?

jjgod