@Test(expected = IllegalStateException.class)
    public void shouldThrowForMultipleByModulators() {
        __.groupCount().by("name").by("age");
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/GroupCountStepTest.java