code: @Override
    public boolean containsKey(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
      Object value = deltaMap.get(key);
      if (value == null) {
        return originalMap.containsKey(key);
      }
      if (value == removedValue) {
        return false;
      }
      return true;
    }
docs: This is more expensive.
  
  @param key key whose presence in this map is to be tested.
  @return <tt>true</tt> if this map contains a mapping for the specified
  key.