@Override
    public TraversalMapStep<S, E> clone() {
        final TraversalMapStep<S, E> clone = (TraversalMapStep<S, E>) super.clone();
        clone.mapTraversal = this.mapTraversal.clone();
        return clone;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/TraversalMapStep.java