code: def mass_2d_lens(self, r, sigma0, Rs):
          
          X = r/Rs
          alpha_r = 2*sigma0 * Rs * X * (1-self._F(X)) / (X**2-1)
          mass_2d = alpha_r * r * np.pi
          return mass_2d
docs: mass enclosed projected 2d sphere of radius r
          :param r:
          :param rho0:
          :param a:
          :param s:
          :return: