code: public void register(ReloadableFile reloadableFile, FileSubscriber fileSubscriber, Identification identification)
                                                                                              throws IllegalIDException {
          IdentificationSet<FileSubscriber> subscribers = fileSubscribers.get(reloadableFile);
  
          if (subscribers == null) {
              subscribers = new IdentificationSet<>(false);
              fileSubscribers.put(reloadableFile, subscribers);
          }
          subscribers.add(fileSubscriber, identification);
      }
docs: Registers a {@link FileSubscriber} with a {@link ReloadableFile}. So when the {@code reloadableFile} is reloaded,
  the fileSubscriber will be notified. Multiple file subscribers can be registered with the same reloadable file.
  
  @param reloadableFile the reloadable file that should be observed
  @param fileSubscriber the fileSubscriber that should be notified when the reloadable file is reloaded
  @param identification the Identification of the FileSubscriber
  @throws IllegalIDException not yet implemented