@Override
    public void addChildOption(final Merge token, final Traversal.Admin<S, Object> traversalOption) {
        if (token == Merge.outV) {
            this.outVTraversal = this.integrateChild(traversalOption);
        } else if (token == Merge.inV) {
            this.inVTraversal = this.integrateChild(traversalOption);
        } else {
            super.addChildOption(token, traversalOption);
        }
    }
Gets the traversal that will be used to provide the {@code Map} that will be used to identify the Direction.IN
vertex during merge.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeEdgeStep.java