for (selector, callback) {
		let slides = $$(`.slide:has(${ selector})`);

		for (let slide of slides) {
			_.on(slide).then(slide => {
				for (let element of $$(selector, slide)) {
					callback(element);
				}
			});
		}

		return slides;
	}
Run code for specific elements, once, only when the slide they appear in becomes active
@param {string} selector
@param {function} callback
@returns {HTMLElement[]} The slides that contain elements matching the selector