code: public static YearMonth randomYearMonthBefore(YearMonth before) {
      checkArgument(before != null, "Before must be non-null");
      LocalDate start = before.atDay(1);
      LocalDate localDate = randomLocalDateBefore(start);
      return YearMonth.of(localDate.getYear(), localDate.getMonth());
    }
docs: Returns a random {@link YearMonth} that is before the given {@link YearMonth}.
  
  @param before the value that returned {@link YearMonth} must be before
  @return the random {@link YearMonth}
  @throws IllegalArgumentException if before is null or if before is equal to or before {@link
  RandomDateUtils#MIN_INSTANT}