def gravity_expected_score(other_mean)
      g = 1 / Math.sqrt(1 + 3 * sd**2 / Math::PI**2)
      [g, 1 / (1 + Math.exp(-g * (other_mean - mean)))]
    end
def gravity_expected_score(other_mean)
      g = 1 / Math.sqrt(1 + 3 * sd**2 / Math::PI**2)
      [g, 1 / (1 + Math.exp(-g * (other_mean - mean)))]
    end
Calculate `g(phi)` and `E(mu, mu_j, phi_j)` as defined in the Glicko2 paper