code: static ClockNormalizer ofOnlyMode(ClockUnit unit) {
  
          ClockNormalizer ret = MAP_ONLY.get(unit);
  
          if (ret == null) {
              throw new IllegalArgumentException(unit.name());
          }
  
          return ret;
  
      }
docs: <p>Liefert einen passenden Normalisierer. </p>
  
  @param   unit    clock unit
  @return  unique normalizer instance