function getSlidePastCount( slide = currentSlide ) {

		let horizontalSlides = getHorizontalSlides();

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

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

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

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

				// Stop as soon as we arrive at the present
				if( verticalSlides[j] === slide ) {
					break mainLoop;
				}

				// Don't count slides with the "uncounted" class
				if( verticalSlides[j].dataset.visibility !== 'uncounted' ) {
					pastCount++;
				}

			}

			// Stop as soon as we arrive at the present
			if( horizontalSlide === slide ) {
				break;
			}

			// Don't count the wrapping section for vertical slides and
			// slides marked as uncounted
			if( horizontalSlide.classList.contains( 'stack' ) === false && horizontalSlide.dataset.visibility !== 'uncounted' ) {
				pastCount++;
			}

		}

		return pastCount;

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

@param {HTMLElement} [slide=currentSlide] The slide we're counting before

@return {number} Past slide count