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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def Nstars(self):
 """ dictionary of number of stars per system """

        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