In this example, a RWN ntuple is duplicated and filled with a subset of the original one. The subset is: ``the people with at least 3 childrens''.

A sub-Ntuple with children >= 3
                                                                           
      opt stat                                                                                                          
      hi/file 1 rwn_aptuple.hbook                                                                                       
1     HI/FILE 2 RWN_APTUPLE_DUP.HBOOK ! N                                                                        
2     NT/DUPL //LUN1/10 100                                                                                      
      *                                                                                                                 
      application comis quit                                                                                            
            real function dup(dum)                                                                                      
4           INCLUDE ?                                                                                            
5           IF (CHILDREN.GE.3) CALL HFN(100,CATEGORY)                                                            
            dup=1.                                                                                                      
            end                                                                                                         
      Quit                                                                                                              
      *                                                                                                                 
3     NT/LOOP //LUN1/10 dup                                                                                      
      hrout 100                                                                                                         
      nt/plot //lun1/10.children                                                                                        
      set hcol 1105                                                                                                     
      nt/plot //lun2/100.children option=s                                                                              

1. Create a new HBOOK file to store the ``subset-Ntuple''
2. Duplicate the structure of the original Ntuple. The subset will be stored in this duplicated ntuple.
3. Loop over the original Ntuple with the function ``dup''.
4. The special COMIS statement avoid to do UWFUNC .
5. When an event is selected, HFN is called to fill the ``subset-Ntuple''