code: static Chronology registerChrono(Chronology chrono, String id) {
          Chronology prev = CHRONOS_BY_ID.putIfAbsent(id, chrono);
          if (prev == null) {
              String type = chrono.getCalendarType();
              if (type != null) {
                  CHRONOS_BY_TYPE.putIfAbsent(type, chrono);
              }
          }
          return prev;
      }
docs: Register a Chronology by ID and type for lookup by {@link #of(String)}.
  Chronos must not be registered until they are completely constructed.
  Specifically, not in the constructor of Chronology.
  
  @param chrono the chronology to register; not null
  @param id the ID to register the chronology; not null
  @return the already registered Chronology if any, may be null