@Override
    public Traverser.Admin<E> processNextStart() {
        if (this.done)
            throw FastNoSuchElementException.instance();
        this.processAllStarts();
        this.done = true;
        return this.getTraversal().asAdmin().getTraverserGenerator().generate(this.supply(), (Step<E, E>) this, 1l);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/SupplyingBarrierStep.java