@Override
        public Traversal<Vertex, Map<String, Map<String, Number>>> get_g_V_outXfollowedByX_group_byXsongTypeX_byXbothE_group_byXlabelX_byXweight_sumXX() {
            return g.V().out("followedBy").<String, Map<String, Number>>group().by("songType").by(bothE().group().by(T.label).by(values("weight").sum()));
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/GroupTest.java