@Override
    public void modulateBy(final Traversal.Admin<?, ?> sackTraversal) {
        if (this.sackTraversal != null) {
            throw new IllegalStateException("Sack step can only have one by modulator");
        }
        this.sackTraversal = this.integrateChild(sackTraversal);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/SackValueStep.java