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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 variance(self, param):
 """ Return the variance in a given parameter as found by the fit. :param param: ``Parameter`` Instance. :return: Variance of ``param``. """

        param_number = self.model.params.index(param)
        try:
            return self.covariance_matrix[param_number, param_number]
        except TypeError:
            # covariance_matrix can be None
            return None