@Override
    public MergeVertexStep<S> clone() {
        return (MergeVertexStep<S>) super.clone();
    }
Implementation for the {@code mergeV()} step covering both the start step version and the one used mid-traversal.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeVertexStep.java