@Override
    public void modulateBy(final Traversal.Admin<?, ?> selectTraversal) {
        if (null == valueTraversal) {
            this.valueTraversal = this.integrateChild(selectTraversal);
        } else {
            throw new IllegalArgumentException("valueMap()/propertyMap() step can only have one by modulator");
        }
    }
@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