@Override
    public List<Traversal.Admin<S, B>> getLocalChildren() {
        return null == this.sackTraversal ? Collections.emptyList() : Collections.singletonList(this.sackTraversal);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/SackValueStep.java