code: public static void addCustomLevel(Level level, String id) {
          if (level == null) {
              throw new IllegalArgumentException("Parameter level can not be 'null' in this call");
          }
          if (id == null) {
              id = level.getName().substring(0, 1);
          }
          customLevels.put(level, id);
      }
docs: Adds level to be treated specially by this formatter.
  
  @param level level to be added to the special list
  @param id one character id to use for this level. If it's <code>null</code> first character of the level name will be used