|| validSurrogatePairAt(b, b.length() - s - 1)) {
      s--;
    }
    return a.subSequence(a.length() - s, a.length()).toString();
  }

  /**
   * True when a valid surrogate pair starts at the given {@code index} in the given {@code string}.
   * Out-of-range indexes return false.
   */
  @VisibleForTesting
  static boolean validSurrogatePairAt(CharSequence string, int index) {
    return index >= 0

        || validSurrogatePairAt(b, b.length() - s - 1)) {
      s--;
    }
    return a.subSequence(a.length() - s, a.length()).toString();
  }

  /**
   * True when a valid surrogate pair starts at the given {@code index} in the given {@code string}.
   * Out-of-range indexes return false.
   */
  @VisibleForTesting
  static boolean validSurrogatePairAt(CharSequence string, int index) {
    return index >= 0