@Override
    public void map(final Vertex vertex, final MapEmitter<Serializable, Long> emitter) {
        final Property<Serializable> cluster = vertex.property(PeerPressureVertexProgram.CLUSTER);
        if (cluster.isPresent()) {
            emitter.emit(cluster.value(), 1l);
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/clustering/peerpressure/ClusterPopulationMapReduce.java