public TraverserSet<Object> combine(final TraverserSet<Object> messageA, final TraverserSet<Object> messageB) {
        messageA.addAll(messageB);
        return messageA;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/traversal/TraversalVertexProgramMessageCombiner.java