@Override
        public Traversal<Vertex, Map<Integer, Collection<Vertex>>> get_g_V_group_byXageX() {
            return g.V().<Integer, Collection<Vertex>>group().by("age");
        }
@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