docs: public final Object putIfAbsent(Object key,
                                      Object value)
      {
          Object foundObject = null;
          java.util.Map subMap = getSubMap(key);
          synchronized (subMap) {
              foundObject = subMap.get(key);
              if (foundObject == null)
                  subMap.put(key,
                             value);
  //      // Assume that most of the time the key is vacant, but be prepared to put back the original 
  //      // if it is not.
  //      foundObject = subMap.put(key,
  //                               value);
  //      // If there was already something there put back the original.
  //      if (foundObject != null && foundObject != value)
  //        subMap.put(key,
  //                   foundObject);
          } // synchronized (subMap).
          return foundObject;
      }