function setPreviousVerticalIndex( stack, v ) {

		if( typeof stack === 'object' && typeof stack.setAttribute === 'function' ) {
			stack.setAttribute( 'data-previous-indexv', v || 0 );
		}

	}
Stores the vertical index of a stack so that the same
vertical slide can be selected when navigating to and
from the stack.

@param {HTMLElement} stack The vertical stack element
@param {string|number} [v=0] Index to memorize