zdoc/assets/sass/abstracts/_functions.scss

@function z($name) {
  @if index($z-indexes, $name) {
    @return (length($z-indexes) - index($z-indexes, $name)) + 1;
  } @else {
    @warn 'There is no item "#{$name}" in this list; choose one of: #{$z-indexes}';
    @return null;
  }
}

@function themed($key) {
  @return map-get($theme-map, $key);
}