@Override
    public CallbackRegistry<Event> getMutatingCallbackRegistry() {
        if (null == callbackRegistry) callbackRegistry = new ListCallbackRegistry<>();
        return callbackRegistry;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/DropStep.java