[Xy-pic] Positioning brace frames

[Scott Morrison]

I'm trying to put some labelled braces in my diagrams, but having
trouble with reference points, etc.

At the moment the best I can do is

(0,0); (10,0) \frm{^\}}

The problem is here the reference point is at (0,0), so xy-pic draws the
nib of the brace above (0,0), and as a result the brace extends further
to the left. How can I specify the reference point arbitrarily?

Additionally, I want to place some text (a single letter) immediately
above the nib. How do I do this?

Thanks,
Scott Morrison