createTractor(document) {
		const tractors = document.querySelectorAll('mlt>tractor[id^="tractor"]');
		const lastTractor = tractors.item(tractors.length - 1);
		const lastID = (lastTractor === null) ? -1 : lastTractor.id.match(/tractor(\d+)/)[1];

		const newTractor = document.createElement('tractor');
		newTractor.id = 'tractor' + (Number(lastID) + 1);
		const videotrack0 = document.getElementById('videotrack0');
		videotrack0.parentElement.insertBefore(newTractor, videotrack0);
		return newTractor;
	}
Create tractor and put it before videotrack0

@param {Document} document
@return {Element} new tractor element