@Override
    public boolean hasNextBarrier() {
        this.processAllStarts();
        return !this.traverserSet.isEmpty();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/CollectingBarrierStep.java