@Override
    public void addChildOption(final Pick pickToken, final Traversal.Admin<S, E> traversalOption) {
        if (Pick.any != pickToken)
            throw new IllegalArgumentException("Union step only supports the any token: " + pickToken);
        super.addChildOption(pickToken, traversalOption);
    }
A placeholder object used to kick off the first traverser in the union.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/branch/UnionStep.java