optional-element($this, $element, $substitute_selector = '', $only_optional = false)
  $selector = $element in $this.elements ? $this.elements[$element] : ''

  if $selector
    $root_selector = stylobate-root($this).selector
    $rooted_selector = stylobate-to-selector($this, $this.selector)
    if $rooted_selector == $root_selector
      $selector = '/' + stylobate-selector($this, '__' + $element)
    else
      $selector = '/' + stylobate-selector($this, replace('ROOT', '__' + $element, $rooted_selector))

  // FIXME: not the best way -_-
  if $substitute_selector
    $selector = '/' + stylobate-selector($this, $substitute_selector)

  rules()
    {block}

  rules() unless $selector or $only_optional
  {$selector}
    rules() if $selector