MACRO y2006h library=$STAR_LIB root=1

   DETP geom y2006h
   make geometry
   gclos all

   Application DATA gstar_upart.g
     Module gstar_upart "Sets W+ --> e+ nu as the only allowed decay for W+"
     Author JCW
     Created 02/01/2010
+CDE,agecom,gconst.
     Particle W_plus_enu  code=42 trktyp=4 mass=80.398 charge=1.0  tlife=0.31600E-24 pdg=24,
              bratio={ 1.0, },
              mode={ 02 04, }
     END
   gstar_upart.g
   gexec gstar_upart.g
   **************************************************

   * usual starsim stuff...
RETURN

     mode = CC BB AA

where AA is the geant ID of the first decay daughter, BB is the geant ID of the second decay daughter and CC is the geant ID of the third decay daughter (if applicable).