🚀 go-pugleaf

I'm having problems with the alignment of a long equation in the eqnarray
mode.  The problem code is as follows:

\documentstyle[12pt]{article}
\begin{document}
\begin{eqnarray}
F(I) & = & a I -  \left[ \frac{1}{I_s^A} +
			\frac{(1-a)}{I_s^B} -
			2\frac{(1-a)}{I_s^C}\right] I^2+
		 \nonumber \\
   &   &          \left[ \frac{1}{{I_s^A}^2} +
			\frac{(1-a)}{{I_s^B}^2} -
			2\frac{(1-a)}{{I_s^C}^2}\right]
			I^3 + \ldots
\end{eqnarray}
\end{document}

I wish to align the second \left[ with the first \left[ rather than the
`aI' as it stands above.  Why can't I simply add a & before the first \left[
and another & in front of the second \left[?

Thanks for your help.
 
-- 

Darran Edmundson
darran@chaos.phys.sfu.ca