@Override
    public void setTraversal(final Traversal.Admin<?, ?> parentTraversal) {
        super.setTraversal(parentTraversal);
        if (valueTraversal != null) { this.integrateChild(this.valueTraversal); }
    }
A map step that returns the merger of the traverser and the provided arguments without duplicates. This is commonly
known as a union.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeStep.java