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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 obfn_fvarf(self):
 """Variable to be evaluated in computing data fidelity term, depending on 'fEvalX' option value. """


        return self.Xf if self.opt['fEvalX'] else \
            sl.rfftn(self.Y, None, self.cri.axisN)