I have a cluster with which the objects are simply bullets. I would like to group some of the bullets such as the following.
Somehow the edges of the box are exactly on the bullets. Here is my code
\[
\xymatrix@C=1cm@R=1cm{
*={\bullet}\ar@{-}[r]&*={\bullet}\ar@{-}[r]&*={\bullet}\ar@{-}[r]\ar@{-}[d]&*={\bullet}
\save "1,1"."2,3"*[F.]\frm{}\\
*={\bullet}\ar@{-}[r]&*={\bullet}\ar@{-}[r]&*={\bullet}\ar@{-}[r]&*={\bullet}
\restore}%
\]
which resulting the following image.
I just need to insert paddings within the box. Or if anybody know another way to draw this with xymatrix would also be great.
Thank you in advance.



