code: @Override  // override with covariant return type
      public Period period(int years, int months, int days) {
          return Period.of(years, months, days);
      }
docs: Obtains a period for this chronology based on years, months and days.
  <p>
  This returns a period tied to the ISO chronology using the specified
  years, months and days. See {@link Period} for further details.
  
  @param years  the number of years, may be negative
  @param months  the number of years, may be negative
  @param days  the number of years, may be negative
  @return the period in terms of this chronology, not null
  @return the ISO period, not null