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