@Override
    public void addBarrier(final TraverserSet<S> barrier) {
        barrier.forEach(traverser -> traverser.setSideEffects(this.getTraversal().getSideEffects()));
        this.traverserSet.addAll(barrier);
        this.barrierConsumed = false;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/CollectingBarrierStep.java