code: @Override
      public Date firstBetween(Date start, Date end) {
          return rule.firstBetween(start, end);
      }
docs: Return the first occurrence of this holiday that is on or after
  the given start date and before the given end date.
  
  @param start Only occurrences on or after this date are returned.
  @param end   Only occurrences before this date are returned.
  
  @return      The date on which this event occurs, or null if it
  does not occur between the start and end dates.
  
  @see #firstAfter
  @hide draft / provisional / internal are hidden on Android