@Override
    public Boolean getEmptyBarrier() {
        // if the barrier is empty then it must be "done"
        return true;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/SideEffectCapStep.java