[XeTeX] math fonts in beamer documents

[VAFA KHALIGHI]

if we make a beamer document like

%%%%%%%%%%%%%%%%%
\documentclass{beamer}
\usepackage{unicode-math}
\setmathfont{Cambria Math}
\begin{document}
\begin{frame}
$$1234$$
\end{frame}
\end{document}
%%%%%%%%%%%%%%%%%%%%%%%


Then the maths font does not change to Cambria Math, but if we just put
\setmathfont{Cambria Math} after \begin{document}, then it changes the math
font to Cambria Math. Obviously beamer is doing some things here which at
the moment I do not understand. My question is what can I do so that in the
example above the maths font will be Cambria Math ? (of course without
putting \setmathfont{Cambria Math} after \begin{document})

Thanks
-- 
``Life is not empty,
There is kindness, there is apple and there is faith

One day will come,
and to a mendicant I will endow a jasmine''

-- Sohrab Sepehri
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