@Override
    public boolean add(final Traverser.Admin<S> traverser) {
        final Traverser.Admin<S> existing = this.map.get(traverser);
        if (null == existing) {
            this.map.put(traverser, traverser);
            return true;
        } else {
            existing.merge(traverser);
            return false;
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/traverser/util/TraverserSet.java