code: function(obj, name, callback, context, listening) {
      obj._events = eventsApi(onApi, obj._events || {}, name, callback, {
        context: context,
        ctx: obj,
        listening: listening
      });
  
      if (listening) {
        var listeners = obj._listeners || (obj._listeners = {});
        listeners[listening.id] = listening;
      }
  
      return obj;
    }
docs: Guard the `listening` argument from the public API.