public @Nullable Void call() throws Exception {
                  barrier.await();
                  Integer first = graph.nodes().iterator().next();
                  for (Integer node : graph.nodes()) {
                    Set<Integer> unused = graph.successors(node);
                  }
                  /*
                   * Also look up an earlier node so that, if the graph is using MapRetrievalCache,

                public @Nullable Void call() throws Exception {
                  barrier.await();
                  Integer first = graph.nodes().iterator().next();
                  for (Integer node : graph.nodes()) {
                    Set<Integer> unused = graph.successors(node);
                  }
                  /*
                   * Also look up an earlier node so that, if the graph is using MapRetrievalCache,