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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 sky_fraction(self):
 """ Sky fraction covered by the MOC """

        pix_id = self._best_res_pixels()
        nb_pix_filled = pix_id.size
        return nb_pix_filled / float(3 << (2*(self.max_order + 1)))