@Override
    public List<Traversal.Admin<?, ?>> getLocalChildren() {
        final List<Traversal.Admin<?, ?>> children = new ArrayList<>(2);
        if (null != this.keyTraversal)
            children.add(this.keyTraversal);
        children.add(this.valueTraversal);
        return children;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/GroupSideEffectStep.java