code: public SafeHtmlBuilder appendContent(Iterable<SafeHtml> htmls) {
      checkNotVoidElement();
      for (SafeHtml html : htmls) {
        contents.add(html);
      }
      return this;
    }
docs: Appends the given {@code htmls} as this element's content, in the sequence the Iterable returns
  them.
  
  @throws IllegalStateException if this builder represents a void element