code: private void setCustomArgStartFormat(int argStart, Format formatter) {
          setArgStartFormat(argStart, formatter);
          if (customFormatArgStarts == null) {
              customFormatArgStarts = new HashSet<Integer>();
          }
          customFormatArgStarts.add(argStart);
      }
docs: Sets a custom formatter for a MessagePattern ARG_START part index.
  "Custom" formatters are provided by the user via setFormat() or similar APIs.