[XeTeX] problem with xetex/fontspec

[Sun w]

Dear All,
 
I have trouble with xetex/fontspec.  The following example shows that it might select a wrong font.
Attached please find a pdf and a log file for this example.
 
Note that the problem disappear if the macro \get at external@font is removed.
 
Best,
Wenchang
 
======================================
% -*- coding: utf-8 -*-
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\makeatletter
 \XeTeXinterchartokenstate=1
 \XeTeXcharclass `A=1
 \def\test{\fontspec{Arial}}
\XeTeXinterchartoks 0 1{\bgroup\test\get at external@font}
\XeTeXinterchartoks 1 0{\egroup}
\bfseries\itshape aAa
\bfseries\slshape aAa !!!
\XeTeXinterchartoks 0 1{\bgroup\test}
\bfseries\itshape aAa
\bfseries\slshape aAa
\end{document}
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