code: function (trackIndex, animation, loop, delay) {
  		var entry = new spine.TrackEntry();
  		entry.animation = animation;
  		entry.loop = loop;
  		entry.endTime = animation.duration;
  
  		var last = this._expandToIndex(trackIndex);
  		if (last) {
  			while (last.next)
  				last = last.next;
  			last.next = entry;
  		} else
  			this.tracks[trackIndex] = entry;
  
  		if (delay <= 0) {
  			if (last)
  				delay += last.endTime - this.data.getMix(last.animation, animation);
  			else
  				delay = 0;
  		}
  		entry.delay = delay;
  
  		return entry;
  	}
docs: Adds an animation to be played delay seconds after the current or last queued animation.
  @param delay May be <= 0 to use duration of previous animation minus any mix duration plus the negative delay.