[Xy-pic] Centring brackets

[Damjan Perenic]

I have following xymatrix code

       \xymatrix at C=2pc @R=0.5pc {
          & \pws{011}{\tar} \\
          \pws{001}{\tar} & \pws{012}{\tar} \save
[d].[u]*++[F\{][F\}]\frm{}\restore & \pws{002}{\tar} \\
          & \pws{003}{\tar}
         }

Which produces output as seen on this picture:
http://dl.dropbox.com/u/5378579/nibs.png

My question is, how can I get brackets' "nibs" centred? I would like
to have them near square 012. I read xypic guide and read the answer
to exercise 21, but I did not get the trick.


Damjan