code: function(slide) {
  		var slide_id = slide.unique_id;
  		if (!TL.Util.trim(slide_id)) {
  			// give it an ID if it doesn't have one
  			slide_id = (slide.text) ? TL.Util.slugify(slide.text.headline) : null;
  		}
  		// make sure it's unique and add it.
  		slide.unique_id = TL.Util.ensureUniqueKey(this.event_dict,slide_id);
  		return slide.unique_id
  	}
docs: Given a slide, verify that its ID is unique, or assign it one which is.
  The assignment happens in this function, and the assigned ID is also
  the return value. Not thread-safe, because ids are not reserved
  when assigned here.