return new VerticalLinearTransformation(x1);
      } else {
        return withSlope((y2 - y1) / (x2 - x1));
      }
    }

    /**
     * Finish building an instance with the given slope, i.e. the rate of change of {@code y} with
     * respect to {@code x}. The slope must not be {@code NaN}. It may be infinite, in which case
     * the transformation is vertical. (If it is zero, the transformation is horizontal.)
     */