* #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();

   *
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset have zero population variance
   */

   *
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset must have zero population variance
   */

  /**
   * Converts the specified {@code String str} from this format to the specified {@code format}. A
   * "best effort" approach is taken; if {@code str} does not conform to the assumed format, then
   * the behavior of this method is undefined but we make a reasonable effort at converting anyway.
   */
  public final String to(CaseFormat format, String str) {
    checkNotNull(format);
    checkNotNull(str);

    assertThat(function.apply("Null")).isNull();

    new EqualsTester()
        .addEqualityGroup(function, Functions.forMap(map, 42))
        .addEqualityGroup(Functions.forMap(map))
        .addEqualityGroup(Functions.forMap(map, null))
        .addEqualityGroup(Functions.forMap(map, 43))
        .testEquals();
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester

  /**
   * Converts the specified {@code String str} from this format to the specified {@code format}. A
   * "best effort" approach is taken; if {@code str} does not conform to the assumed format, then
   * the behavior of this method is undefined but we make a reasonable effort at converting anyway.
   */
  public final String to(CaseFormat format, String str) {
    checkNotNull(format);
    checkNotNull(str);

 * <td>{@link DateFormat#SHORT} format</td>
 * <td>{@literal H:mm}</td>
 * </tr>
 * <tr>
 * <td>{@link DateFormat#MEDIUM} format</td>
 * <td>{@literal H:mm:ss}</td>
 * </tr>
 * <tr>
 * <td>{@link DateFormat#LONG} format</td>
 * <td>{@literal H:mm:ss z}</td>
 * </tr>
 * <tr>
 * <td>{@link DateFormat#FULL} format</td>
 * <td>{@literal H'時'mm'分'ss'秒' z}</td>
 * </tr>
 * <tr>

        SEARCH,
        /** Label API format. */
        LABEL,
        /** Popular word API format. */
        POPULARWORD,
        /** Favorite API format. */
        FAVORITE,
        /** Favorites API format. */
        FAVORITES,
        /** Ping API format. */
        PING,
        /** Scroll API format. */
        SCROLL,
        /** Suggest API format. */
        SUGGEST,
        /** Other API format. */

    assertThat(function.apply("Null")).isNull();

    new EqualsTester()
        .addEqualityGroup(function, Functions.forMap(map, 42))
        .addEqualityGroup(Functions.forMap(map))
        .addEqualityGroup(Functions.forMap(map, null))
        .addEqualityGroup(Functions.forMap(map, 43))
        .testEquals();
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester

     * @param format the date format pattern or "pdf_date" for PDF date format
     * @return parsed Date object, or null if parsing fails
     */
    public static Date parseDate(final String value, final String format) {
        if (value == null) {
            return null;
        }

        try {
            if (PDF_DATE.equals(format)) {
                final Calendar cal = DateConverter.toCalendar(value);