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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 get_model(self, sm_id):
 """ Extract a CompositeSourceModel instance containing the single model of index `sm_id`. """

        sm = self.source_models[sm_id]
        if self.source_model_lt.num_samples:
            self.source_model_lt.num_samples = sm.samples
        new = self.__class__(self.gsim_lt, self.source_model_lt, [sm],
                             self.optimize_same_id)
        new.sm_id = sm_id
        return new