select(el) {
				let sortable = el.parentNode[expando];
				if (!sortable || !sortable.options.multiDrag || ~multiDragElements.indexOf(el)) return;
				if (multiDragSortable && multiDragSortable !== sortable) {
					multiDragSortable.multiDrag._deselectMultiDrag();
					multiDragSortable = sortable;
				}
				toggleClass(el, sortable.options.selectedClass, true);
				multiDragElements.push(el);
			}
Selects the provided multi-drag item
@param  {HTMLElement} el    The element to be selected