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