  RooDataSet d12("d12","d12",w::x,Index(w::index),Import("S",*d1),Import("B",*d2))

   //-----------------------------------------------------------------------------
   // NEW: make virtual composite dataset (input data is linked, no data is copied)
   RooDataSet d12a("d12a","d12a",w::x,Index(w::index),Link("S",*d1),Link("B",*d2))
   //-----------------------------------------------------------------------------

   // Fit composite dataset to dummy model
   w-&gt;factory("SUM::model(fsig[0,1]*g,u)")
   w: