/// Get a z-index from a map.
///
/// @group 02-z-index
///
/// @param  {String} $index        - The index to get.
/// @param  {Map}    $z [$z-index] - The map containing z-indeces.
/// @return {Number}               - The corresponding z-index value.
@function z($index, $z: $z-index) {
  @if not $z {
    @error 'Value passed for $map not found!';
  }
  @if not map-get($z, $index) {
    @error 'No index `#{inspect($index)}` found!';
  }
  @return map-get($z, $index);
}