docs: public static final void clear(TitanGraph graph) {
          Preconditions.checkNotNull(graph);
          Preconditions.checkArgument(graph instanceof StandardTitanGraph,"Invalid graph instance detected: %s",graph.getClass());
          StandardTitanGraph g = (StandardTitanGraph)graph;
          Preconditions.checkArgument(!g.isOpen(),"Graph needs to be shut down before it can be cleared.");
          final GraphDatabaseConfiguration config = g.getConfiguration();
          BackendOperation.execute(new Callable<Boolean>(){
              @Override
              public Boolean call() throws Exception {
                  config.getBackend().clearStorage();
                  return true;
              }
              @Override
              public String toString() { return "ClearBackend"; }
          }, Duration.ofSeconds(20));
      }