@Override
    public void setTraversal(final Traversal.Admin<?, ?> parentTraversal) {
        super.setTraversal(parentTraversal);
        if (null != this.propertyTraversal)
            this.integrateChild(this.propertyTraversal);
        if (null != this.valueTraversal)
            integrateChild(this.valueTraversal);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/PropertyMapStep.java