docs: private static DateTimeFormatter createFormatterForStyle(String style) {
          if (style == null || style.length() != 2) {
              throw new IllegalArgumentException("Invalid style specification: " + style);
          }
          int dateStyle = selectStyle(style.charAt(0));
          int timeStyle = selectStyle(style.charAt(1));
          if (dateStyle == NONE && timeStyle == NONE) {
              throw new IllegalArgumentException("Style '--' is invalid");
          }
          return createFormatterForStyleIndex(dateStyle, timeStyle);
      }