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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 _to_use(self):
 """ Returns a tensor in the USE reference frame """

        if self.ref_frame is 'NED':
            # Rotate
            return utils.ned_to_use(self.tensor), \
                utils.ned_to_use(self.tensor_sigma)
        elif self.ref_frame is 'USE':
            # Already USE
            return self.tensor, self.tensor_sigma
        else:
            raise ValueError('Reference frame %s not recognised - cannot '
                             'transform to USE!' % self.ref_frame)