function getPreviousVerticalIndex( stack ) {

		if( typeof stack === 'object' && typeof stack.setAttribute === 'function' && stack.classList.contains( 'stack' ) ) {
			// Prefer manually defined start-indexv
			const attributeName = stack.hasAttribute( 'data-start-indexv' ) ? 'data-start-indexv' : 'data-previous-indexv';

			return parseInt( stack.getAttribute( attributeName ) || 0, 10 );
		}

		return 0;

	}
Retrieves the vertical index which was stored using
#setPreviousVerticalIndex() or 0 if no previous index
exists.

@param {HTMLElement} stack The vertical stack element