code: private static void appendTag(String tag, StringBuilder buffer) {
          if (buffer.length() != 0) {
              buffer.append(UNDERSCORE);
          }
  
          buffer.append(tag);
      }
docs: Append a tag to a StringBuilder, adding the separator if necessary.The tag must
  not be a zero-length string.
  
  @param tag The tag to add.
  @param buffer The output buffer.