code: def cnst_A0(self, X, Xf=None):
          r
  
          # This calculation involves non-negligible computational cost
          # when Xf is None (i.e. the function is not being applied to
          # self.X).
          if Xf is None:
              Xf = sl.rfftn(X, None, self.cri.axisN)
          return sl.irfftn(sl.inner(self.Df, Xf, axis=self.cri.axisM),
                           self.cri.Nv, self.cri.axisN)
docs: r"""Compute :math:`A_0 \mathbf{x}` component of ADMM problem
          constraint.