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