code: function on(priv, eventName, listener) {
    check(eventName, 'eventName').is.aString.and.oneOf(EVENT_NAMES)();
    check(listener, 'listener').is.aFunction();
  
    getListeners(priv, eventName).push(listener);
  }
docs: Registers a listener on given eventName.
  
  @param {String} eventName name of event
  @param {Function} listener a function
  @postcondition given listener will be notified about current slide changes
  @fqn Slider.prototype.on