* @since 14.0 (since 12.0 as {@code toImmutableSet()}).
   */
  @SuppressWarnings("nullness") // Unsafe, but we can't do much about it now.
  public final ImmutableSet<@NonNull E> toSet() {
    return ImmutableSet.copyOf((Iterable<@NonNull E>) getDelegate());
  }

  /**
   * Returns an {@code ImmutableSortedSet} containing all of the elements from this {@code

   * @since 14.0 (since 12.0 as {@code toImmutableSet()}).
   */
  @SuppressWarnings("nullness") // Unsafe, but we can't do much about it now.
  public final ImmutableSet<@NonNull E> toSet() {
    return ImmutableSet.copyOf((Iterable<@NonNull E>) getDelegate());
  }

  /**
   * Returns an {@code ImmutableSortedSet} containing all of the elements from this {@code

        if (fieldSet == null) {
            fieldSet = split(getIndexAdminArrayFields(), ",")
                    .get(stream -> stream.filter(StringUtil::isNotBlank).map(String::trim).collect(Collectors.toSet()));
            propMap.put(INDEX_ADMIN_ARRAY_FIELD_SET, fieldSet);
        }
        return fieldSet;
    }

    default boolean validateIndexArrayFields(final Map<String, Object> source) {

        final Set<String> excludeFields = Arrays.stream(fessConfig.getIndexExportExcludeFields().split(","))
                .map(String::trim)
                .filter(s -> !s.isEmpty())
                .collect(Collectors.toSet());
        final int scrollSize = fessConfig.getIndexExportScrollSizeAsInteger();

        final IndexExportFormatter resolvedFormatter =