/**
 * Z-Index function
 *
 * @param {string} $list (Array) - The list to find the current z-index in
 * @param {string} $element (String) - The name of the current element, must be in the $list
 *
 * @returns {number}
 */
@function zindex($list, $element) {
	$zIndex: index($list, $element);

	@if $zIndex {
		@return $zIndex;
	}

	@warn 'There is no item "#{$element}" in this list; choose one of: #{$list}';
	@return null;
}