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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 covariance(self, param_1, param_2):
 """ Return the covariance between param_1 and param_2. :param param_1: ``Parameter`` Instance. :param param_2: ``Parameter`` Instance. :return: Covariance of the two params. """

        param_1_number = self.model.params.index(param_1)
        param_2_number = self.model.params.index(param_2)
        return self.covariance_matrix[param_1_number, param_2_number]