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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 D_dt(self, z_lens, z_source):
 """ time-delay distance :param z_lens: redshift of lens :param z_source: redshift of source :return: time-delay distance in units of Mpc """

        return self.D_xy(0, z_lens) * self.D_xy(0, z_source) / self.D_xy(z_lens, z_source) * (1 + z_lens)