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