:model.fitTo(d12a)
</pre></li>
  For virtual composite dataset it is also possible to join a mix of binned and unbinned datasets
  (representation as a <tt>RooDataSet</tt> with weights)

<li><b>The setWeightVar() method has been deprecated as it is very difficult to support on-the-fly redefinition
       of the event weight variable in the new data store scheme</b>. To declare a data set weighed,
       use the WeightVar() modifier of the constructor instead,e.g.:
<pre>
    RooDataSet wdata("wdata","wdata",