@Override
    public Traverser.Admin<S> poll() {  // pop, null if empty
        return this.map.isEmpty() ? null : this.remove();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/traverser/util/TraverserSet.java