\xymatrix{ ... }
|
where the "..." should be replaced by entries to be aligned in rows and columns where
& and
\\.

was typeset by
\xymatrix{
A &*+[F]{\sum_{i=n}^m {i^2}} \\
& {\bullet} & D \ar[ul] }
Notice the following:
{} around \bullet.
\ar[ul]) in entries allow drawing of
arrows and such relative to the entries without changing the overall
layout, and
*+[F]) first in entries allow changing the
format and shape in many ways.