@Override
    public NoOpBarrierStep<S> clone() {
        final NoOpBarrierStep<S> clone = (NoOpBarrierStep<S>) super.clone();
        clone.barrier = (TraverserSet<S>) this.traversal.getTraverserSetSupplier().get();
        return clone;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/NoOpBarrierStep.java