Label a block or systems of equations with one label

Question

Possible Duplicate:
How to get only one vertically centered equation number in align environment with two equations

I have a big system of equations which I want to label as one block. I do want to use align as the backbone of the typeset. I can produce, using \nonumber, something like

\begin{align}
a &= & b \nonumber\\
x+y & & =z\nonumber\\
w =& v & \nonumber\\
1 & 2 & 3
\label{eq1}
\eng{align}

However, this produces the label at the end, and I guess that a better look would be that the label will be typeset at the middle of the vertical height of the block. I tried to use a combination of \equation and \split but then I don't know how to take advantage of the columns available in \align.

So, how can I achieve this? If you think that this won't look good then what are the other possibilities I can take?

Answer 1

Use the "inner" version of align

\begin{equation}
\begin{aligned}
a &= & b\\
x+y & & =z\\
w =& v & \\
1 & 2 & 3
\end{aligned}
\label{eq1}
\end{equation}