code: def Nstars(self):
          
          if self._Nstars is None:
              N = {}
              for n in self.get_model_nodes():
                  if n.index not in N:
                      N[n.index] = 1
                  else:
                      N[n.index] += 1
              self._Nstars = N
          return self._Nstars
docs: dictionary of number of stars per system