st: Shea's R2 for xtivreg

[iskotida@aueb.gr]

> Hello StataList,
> I am running an xtivreg, re and I am trying to measure the 
> validity of my instruments.  Does anyone know of a way for me 
> to obtain the first stage statistics?  In particular
> I need the Shea's partial R2. 
thanks 
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