code: public YearMonthDay withFieldAdded(DurationFieldType fieldType, int amount) {
          int index = indexOfSupported(fieldType);
          if (amount == 0) {
              return this;
          }
          int[] newValues = getValues();
          newValues = getField(index).add(this, index, newValues, amount);
          return new YearMonthDay(this, newValues);
      }
docs: Returns a copy of this date with the value of the specified field increased.
  <p>
  If the addition is zero, then <code>this</code> is returned.
  <p>
  These three lines are equivalent:
  <pre>
  YearMonthDay added = ymd.withFieldAdded(DurationFieldType.days(), 6);
  YearMonthDay added = ymd.plusDays(6);
  YearMonthDay added = ymd.dayOfMonth().addToCopy(6);
  </pre>
  
  @param fieldType  the field type to add to, not null
  @param amount  the amount to add
  @return a copy of this instance with the field updated
  @throws IllegalArgumentException if the value is null or invalid
  @throws ArithmeticException if the new datetime exceeds the capacity