* #transform} methods all return {@link Double#NaN}. The {@link #inverse} method returns the same
   * instance.
   */
  public static LinearTransformation forNaN() {
    return NaNLinearTransformation.INSTANCE;
  }

  /** Returns whether this is a vertical transformation. */
  public abstract boolean isVertical();

    assertThrows(IllegalArgumentException.class, () -> LinearTransformation.horizontal(Double.NaN));
  }

  public void testForNaN() {
    LinearTransformation transformation = LinearTransformation.forNaN();
    assertLinearTransformationNaN(transformation);
  }

    assertThrows(IllegalArgumentException.class, () -> LinearTransformation.horizontal(Double.NaN));
  }

  public void testForNaN() {
    LinearTransformation transformation = LinearTransformation.forNaN();
    assertLinearTransformationNaN(transformation);
  }