TMVA Multiclass and weight

Dear Rooters and TMVA experts,

     I have a question concerning the method "SetWeightExpression" for a MultiClass MVA.
When calling the method

     factory->SetWeightExpression( "<YourWeightExpression>" ); it calls automatically the method [1]

     SetSignalWeightExpression
and

     SetBackgroundWeightExpression.

But in the MultiClass case I do not have a "signal" and a "background". I attach a code example (that is only a slight modification of the test code found in root/tmva/test/TMVAMulticlass.C [2]) where the problem is reproduced

Is it possible to add a weight for each event also for the MultiClass case?

Thanks,

     Andrea Massironi

[1] http://root.cern.ch/root/htmldoc/src/TMVA__Factory.cxx.html#562
[2] http://root.cern.ch/svn/root/trunk/tmva/test/TMVAMulticlass.C