@Override
    public void setKeepLabels(final Set<String> keepLabels) {
        this.keepLabels = new HashSet<>(keepLabels);
        if (null != this.dedupLabels)
            this.keepLabels.addAll(this.dedupLabels);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MatchStep.java