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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 _spectrum(self, photon_energy):
 """Compute differential IC spectrum for energies in ``photon_energy``. Compute IC spectrum using IC cross-section for isotropic interaction with a blackbody photon spectrum following Khangulyan, Aharonian, and Kelner 2014, ApJ 783, 100 (`arXiv:1310.7971 <http://www.arxiv.org/abs/1310.7971>`_). Parameters photon_energy : :class:`~astropy.units.Quantity` instance Photon energy array. """

        outspecene = _validate_ene(photon_energy)

        self.specic = []

        for seed in self.seed_photon_fields:
            # Call actual computation, detached to allow changes in subclasses
            self.specic.append(
                self._calc_specic(seed, outspecene).to("1/(s eV)")
            )

        return np.sum(u.Quantity(self.specic), axis=0)