code: public Period plusWeeks(int weeks) {
          if (weeks == 0) {
              return this;
          }
          int[] values = getValues();  // cloned
          getPeriodType().addIndexedField(this, PeriodType.WEEK_INDEX, values, weeks);
          return new Period(values, getPeriodType());
      }
docs: Returns a new period plus the specified number of weeks added.
  <p>
  This period instance is immutable and unaffected by this method call.
  
  @param weeks  the amount of weeks to add, may be negative
  @return the new period plus the increased weeks
  @throws UnsupportedOperationException if the field is not supported