void  wtaunu() {
  // initialize W->tau nu

  TG3Pythia6* py = TG3Pythia6::Instance();

  py->SetMSEL(0);
  py->SetMSUB(2,1);
//-----------------------------------------------------------------------------
// now turn off all the channels except W->tau nu
// first decay channel: 190, W->tau nu: #208
//-----------------------------------------------------------------------------
  int nch = py->NDecayChannels("W+");

  for (int i=0; i<nch; i++) {
    int code = py->GetMDME(190+i,1);
    if (code > 0) {
      py->SetMDME(190+i,1,0);
    }
  }
  py->SetMDME(208,1,1);

  py->PrintDecayChannels("W+");
}