@Override
    public void addCallback(final EventCallback<E> c) {
        if (callbacks == null) callbacks = new ArrayList<>();
        callbacks.add(c);
    }
@author Stephen Mallette (http://stephen.genoprime.com)
@author Matt Frantz (http://github.com/mhfrantz)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/event/ListCallbackRegistry.java