code: @Override
    public DurationFormatterFactory setFallback(DateFormatter fallback) {
      boolean doReset = fallback == null
          ? this.fallback != null
          : !fallback.equals(this.fallback);
      if (doReset) {
        this.fallback = fallback;
        reset();
      }
      return this;
    }
docs: Set a fallback formatter for durations over a given limit.
  
  @param fallback the fallback formatter to use, or null
  @return this BasicDurationFormatterFactory