code: public CalendarDate setEra(Era era) {
          if (this.era == era) {
              return this;
          }
          this.era = era;
          normalized = false;
          return this;
      }
docs: Sets the era of the date to the specified era. The default
  implementation of this method accepts any Era value, including
  <code>null</code>.
  
  @exception NullPointerException if the calendar system for this
  <code>CalendarDate</code> requires eras and the specified era
  is null.
  @exception IllegalArgumentException if the specified
  <code>era</code> is unknown to the calendar
  system for this <code>CalendarDate</code>.