public function addIndent($whitespace) {
    $has_indent = $this->children(function (Node $node) {
      return !($node instanceof CommentNode);
    })->count() > 0;
    if ($has_indent) {
      $this->children(Filter::isInstanceOf('\Pharborist\WhitespaceNode'))->each(function (WhitespaceNode $ws_node) use ($whitespace) {
        $ws_node->setText($ws_node->getText() . $whitespace);
      });
    }
    else {
      $this->children()->before(Token::whitespace($whitespace));
    }
    return $this;
  }