tetex for Mac OS X and prosper package

I started to use prosper package recently. 
It seems like there is a bug in tetex for Mac OS X (or tetex in general?). 
The following code works in PC.

\overlays{3}{
\begin{slide}{example}
\begin{eqnarray*}
\onlySlide*{1}{lhs  &=& \phantom{rhs 2} \\}
\fromSlide*{2}{lhs  &=& rhs \phantom{2}\\}
\fromSlide*{3}{\phantom{lhs}  &=& rhs 2\\}
\end{eqnarray*}
\end{slide}
}

However, it does not in tetex for Mac. 
I needed to modify the code as the following to make it work .

\newcommand{\annn}{&}

\overlays{3}{
\begin{slide}{example}
\begin{eqnarray*}
\onlySlide*{1}{lhs  \annn = \annn \phantom{rhs 2} \\}
\fromSlide*{2}{lhs  \annn = \annn rhs \phantom{2}\\}
\fromSlide*{3}{\phantom{lhs}  \annn = \annn rhs 2\\}
\end{eqnarray*}
\end{slide}
}

Isn't it strange?

Joon Song