code: protected Set<Long> getModelTrainingDifference(final DataModelIF<Long, Long> model, final Long user) {
          final Set<Long> items = new HashSet<Long>();
          if (training.getUserItems(user) != null) {
              final Set<Long> trainingItems = new HashSet<>();
              for (Long i : training.getUserItems(user)) {
                  trainingItems.add(i);
              }
              for (Long item : model.getItems()) {
                  if (!trainingItems.contains(item)) {
                      items.add(item);
                  }
              }
          }
          return items;
      }
docs: Get the items appearing in the training set and not in the data model.
  
  @param model The data model.
  @param user The user.
  @return The items not appearing in the training set.