docs: public static SanitizedContent concatHtml(SanitizedContent... contents) {
      for (SanitizedContent content : contents) {
        checkArgument(content.getContentKind() == ContentKind.HTML, "Can only concat HTML");
      }
  
      StringBuilder combined = new StringBuilder();
      Dir dir = Dir.NEUTRAL; // Empty string is neutral.
      for (SanitizedContent content : contents) {
        combined.append(content.getContent());
        if (dir == Dir.NEUTRAL) {
          // neutral + x -> x
          dir = content.getContentDirection();
        } else if (content.getContentDirection() == dir
            || content.getContentDirection() == Dir.NEUTRAL) {
          // x + x|neutral -> x, so leave dir unchanged.
        } else {
          // LTR|unknown + RTL|unknown -> unknown
          // RTL|unknown + LTR|unknown -> unknown
          dir = null;
        }
      }
      return SanitizedContent.create(combined.toString(), ContentKind.HTML, dir);
    }