[Xy-pic] bigoplus

[Olivier Wittenberg]

Hello,

I'm having problems with the following code.

$$
\xymatrix{
0 \ar[r] & \displaystyle \bigoplus_{x \in X} g_x \ar[d] \\
         & M
}
$$

The \bigoplus is not vertically aligned with the horizontal arrow,
as it should be. After some hacking, all I have been able to do
is to write something like this:

**{+<0mm,4.5mm>[d]}{\displaystyle \bigoplus_{x \in X} g_x \ar[d]}

This code makes the vertical arrow start at the wrong place, and
it is bad for other obvious reasons.

I suppose this is a classical question, but I haven't found anything
related to it in the archives.

--Olivier Wittenberg