@Test(expected = IllegalStateException.class)
    public void shouldThrowForMultipleByModulators() {
        __.groupCount("x").by("name").by("age");
    }
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/GroupCountSideEffectStepTest.java