[XeTeX] semantic nest size

[Apostolos Syropoulos]

Hello,
I have this code
documentclass[a4paper]{article}
\usepackage{xltxtra}
\usepackage{xgreek}
\usepackage{unicode-math}
\usepackage{amsmath}
\begin{document}
\setmainfont[Mapping=tex-text,Ligatures=Common]{Garamond Premier Pro}
\setsansfont[Mapping=tex-text,Scale=MatchLowercase]{Myriad Pro}
\setmonofont[Scale=MatchLowercase]{Courier}
\setmathfont[Scale=MatchUppercase]{Asana Math}
\begin{displaymath}
\alpha^{\nu}=\underbrace{\alpha\cdot\alpha\cdots\alpha\cdot\alpha}_{\text{$\nu$ παράγοντες}}.
\end{displaymath}
\end{document}

I try to process it with XeLaTeX but it fails:
! TeX capacity exceeded, sorry [semantic nest size=500].
<recently read> {
                 
l.14 ...{\alpha\cdot\alpha\cdots\alpha\cdot\alpha}
                                                  _{\text{$\nu$ παράγοντες}}.
No pages of output.

When I have 
\usepackage{amsmath}
\usepackage{unicode-math}
It compiles just fine. What is wrong? Any ideas?

A.S.
----------------------
Apostolos Syropoulos
Xanthi, Greece
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