class RooExtendPdf: public RooAbsPdf

  RooExtendPdf is a wrappper around an existing PDF that adds a
  parameteric extended likelihood term to the PDF, optionally divided by a
  fractional term from a partial normalization of the PDF:

  nExpected = N   _or Expected = N / frac

  where N is supplied as a RooAbsReal to RooExtendPdf.
  The fractional term is defined as
                          _       _ _   _  _
            Int(cutRegion[x]) pdf(x,y) dx dy
     frac = ---------------_-------_-_---_--_
            Int(normRegion[x]) pdf(x,y) dx dy

        _                                                               _
  where x is the set of dependents involved in the selection region and y
  is the set of remaining dependents.
            _
  cutRegion[x] is an limited integration range that is contained in
  the nominal integration range normRegion[x[]