code: @Override
      public void init() {
          // init super
          super.init();
          // check: at least one search added
          if (searches.isEmpty()) {
              throw new SearchException("Cannot initialize basic parallel search: "
                                      + "no subsearches added for concurrent execution.");
          }
          // initialize subsearches
          searches.parallelStream().forEach(Search::init);
      }
docs: When the search is initialized, it is verified whether at least one subsearch
  has been added and all subsearches are initialized as well (in parallel).
  An exception is thrown if no subsearches have been added.
  
  @throws SearchException if no searches have been added