@Override
    public TraverserSet<S> nextBarrier() throws NoSuchElementException {
        if (!this.starts.hasNext())
            throw FastNoSuchElementException.instance();
        final TraverserSet<S> barrier = (TraverserSet<S>) this.traversal.getTraverserSetSupplier().get();
        while (this.starts.hasNext()) {
            barrier.add(this.starts.next());
        }
        return barrier;
    }
@author Matt Frantz (http://github.com/mhfrantz)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/TailGlobalStep.java