@Override
    public PropertyMapStep<K,E> clone() {
        final PropertyMapStep<K,E> clone = (PropertyMapStep<K,E>) super.clone();
        if (null != this.propertyTraversal)
            clone.propertyTraversal = this.propertyTraversal.clone();
        if (null != this.valueTraversal)
            clone.valueTraversal = this.valueTraversal.clone();
        return clone;
    }
@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