code: public static SafeStyleSheet concat(Iterable<SafeStyleSheet> stylesheets) {
      int concatLength = 0;
      for (SafeStyleSheet stylesheet : stylesheets) {
        concatLength += stylesheet.getSafeStyleSheetString().length();
      }
  
      StringBuilder result = new StringBuilder(concatLength);
      for (SafeStyleSheet stylesheet : stylesheets) {
        result.append(stylesheet.getSafeStyleSheetString());
      }
      return create(result.toString());
    }
docs: Creates a new SafeStyleSheet which contains, in order, the string representations of the given
  {@code stylesheets}.