@Override
        public Traversal<Vertex, Map<String, Number>> get_g_V_group_byXlabelX_byXbothE_groupXaX_byXlabelX_byXweight_sumX_weight_sumX() {
            return g.V().<String, Number>group().by(T.label).by(bothE().group("a").by(T.label).by(values("weight").sum()).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