code: @Override
    public V get(Object key) {
      // key could be not in original or in deltaMap
      // key could be not in original but in deltaMap
      // key could be in original but removed from deltaMap
      // key could be in original but mapped to something else in deltaMap
      V deltaResult = deltaMap.get(key);
      if (deltaResult == null) {
        return originalMap.get(key);
      }
      if (deltaResult == nullValue) {
        return null;
      }
      if (deltaResult == removedValue) {
        return null;
      }
      return deltaResult;
    }
docs: This may cost twice what it would in the original Map.
  
  @param key key whose associated value is to be returned.
  @return the value to which this map maps the specified key, or
  <tt>null</tt> if the map contains no mapping for this key.