def add(other_rating, score)
      g, e = other_rating.gravity_expected_score(rating.mean)
      @v_pre += g**2 * e * (1 - e)
      @delta_pre += g * (score - e)
    end
def add(other_rating, score)
      g, e = other_rating.gravity_expected_score(rating.mean)
      @v_pre += g**2 * e * (1 - e)
      @delta_pre += g * (score - e)
    end
Add an outcome against the rating