@Override
    public ShortestPathVertexProgramStep clone() {
        final ShortestPathVertexProgramStep clone = (ShortestPathVertexProgramStep) super.clone();
        clone.targetVertexFilter = this.targetVertexFilter.clone();
        clone.edgeTraversal = this.edgeTraversal.clone();
        clone.distanceTraversal = this.distanceTraversal.clone();
        return clone;
    }
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/traversal/step/map/ShortestPathVertexProgramStep.java