public static LinearTransformationBuilder mapping(double x1, double y1) {
    checkArgument(isFinite(x1) && isFinite(y1));
    return new LinearTransformationBuilder(x1, y1);
  }

  /**
   * This is an intermediate stage in the construction process. It is returned by {@link
   * LinearTransformation#mapping}. You almost certainly don't want to keep instances around, but