code: public Weeks plus(int weeks) {
          if (weeks == 0) {
              return this;
          }
          return Weeks.weeks(FieldUtils.safeAdd(getValue(), weeks));
      }
docs: Returns a new instance with the specified number of weeks added.
  <p>
  This 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 specified number of weeks
  @throws ArithmeticException if the result overflows an int