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