function getSlidePastCount() {

		var horizontalSlides = toArray( dom.wrapper.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR ) );

		// The number of past slides
		var pastCount = 0;

		// Step through all slides and count the past ones
		mainLoop: for( var i = 0; i < horizontalSlides.length; i++ ) {

			var horizontalSlide = horizontalSlides[i];
			var verticalSlides = toArray( horizontalSlide.querySelectorAll( 'section' ) );

			for( var j = 0; j < verticalSlides.length; j++ ) {

				// Stop as soon as we arrive at the present
				if( verticalSlides[j].classList.contains( 'present' ) ) {
					break mainLoop;
				}

				pastCount++;

			}

			// Stop as soon as we arrive at the present
			if( horizontalSlide.classList.contains( 'present' ) ) {
				break;
			}

			// Don't count the wrapping section for vertical slides
			if( horizontalSlide.classList.contains( 'stack' ) === false ) {
				pastCount++;
			}

		}

		return pastCount;

	}
Returns the number of past slides. This can be used as a global
flattened index for slides.