code: public Object add(final Object value)
  	{
  		final Object key = new IdentityKey(value);
  		if (!super.containsKey(key))
  		{
  			return super.put(key, value);
  		}
  		else
  		{
  			return null;
  		}
  	}
docs: adds an object to the Map. new IdentityKey(obj) is used as key