* eliminate duplicate keys (for buildKeepingLast()) then we have to ensure that a later call to
   * buildOrThrow() will still throw as if the duplicates had not been eliminated. And the exception
   * message must mention two values that were associated with the duplicate key in two different
   * calls to Builder.put (though we don't really care *which* two values if there were more than

   *
   * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
   * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
   * first permutation will be in ascending order, and the last will be in descending order.
   *

   *
   * <p>If you are designing a new system that needs HMAC, prefer {@link #hmacSha256} or other
   * future-proof algorithms <a
   * href="https://datatracker.ietf.org/doc/html/rfc6151#section-2.3">over {@code hmacMd5}</a>.
   *
   * @param key the secret key
   * @throws IllegalArgumentException if the given key is inappropriate for initializing this MAC
   * @since 20.0
   */

   * whose keys and values are the result of applying the provided mapping functions to the input
   * elements.
   *
   * <p>For streams with defined encounter order (as defined in the Ordering section of the {@link
   * java.util.stream} Javadoc), that order is preserved, but entries are <a
   * href="ImmutableMultimap.html#iteration">grouped by key</a>.
   *
   * <p>Example:
   *
   * <pre>{@code

  /**
   * The "base64" base encoding specified by <a
   * href="http://tools.ietf.org/html/rfc4648#section-4">RFC 4648 section 4</a>, Base 64 Encoding.
   * (This is the same as the base 64 encoding from <a
   * href="http://tools.ietf.org/html/rfc3548#section-3">RFC 3548</a>.)
   *
   * <p>The character {@code '='} is used for padding, but can be {@linkplain #omitPadding()

   *
   * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
   * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
   * first permutation will be in ascending order, and the last will be in descending order.
   *

    // places left to rotate.
    int m = -distance % length;
    m = (m < 0) ? m + length : m;
    // The current index of what will become the first element of the rotated section.
    int newFirstIndex = m + fromIndex;
    if (newFirstIndex == fromIndex) {
      return;
    }

    reverse(array, fromIndex, newFirstIndex);
    reverse(array, newFirstIndex, toIndex);

    // places left to rotate.
    int m = -distance % length;
    m = (m < 0) ? m + length : m;
    // The current index of what will become the first element of the rotated section.
    int newFirstIndex = m + fromIndex;
    if (newFirstIndex == fromIndex) {
      return;
    }

    reverse(array, fromIndex, newFirstIndex);
    reverse(array, newFirstIndex, toIndex);

   * whose keys and values are the result of applying the provided mapping functions to the input
   * elements.
   *
   * <p>For streams with defined encounter order (as defined in the Ordering section of the {@link
   * java.util.stream} Javadoc), that order is preserved, but entries are <a
   * href="ImmutableMultimap.html#iteration">grouped by key</a>.
   *
   * <p>Example:
   *
   * <pre>{@code

  /** The HTTP {@code Host} header field name. */
  public static final String HOST = "Host";
  /**
   * The HTTP <a href="https://tools.ietf.org/html/rfc7540#section-3.2.1">{@code HTTP2-Settings}
   * </a> header field name.
   *
   * @since 24.0
   */
  public static final String HTTP2_SETTINGS = "HTTP2-Settings";
  /** The HTTP {@code If-Match} header field name. */