public static <S> Traverser.Admin<S> processTraverserPathLabels(final Traverser.Admin<S> traverser, final Set<String> labels) {
        if (null != labels) traverser.keepLabels(labels);
        return traverser;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/PathProcessor.java