@Override
    public void addBarrier(final Map<Object, Traverser.Admin<S>> barrier) {
        if (null == this.barrier)
            this.barrier = new HashMap<>(barrier);
        else
            this.barrier.putAll(barrier);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/DedupGlobalStep.java