@Override
    public Boolean nextBarrier() throws NoSuchElementException {
        this.processAllStarts();
        this.done = true;
        return true;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/SupplyingBarrierStep.java