@Override
    public void addLocalChild(final Traversal.Admin<?, ?> groupTraversal) {
        this.keyTraversal = this.integrateChild(groupTraversal);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/GroupCountStep.java