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Changes
* Added a property to :class:`boardgamegeek.games.BoardGame`, ``expansion`` which indicates if this item is an expansion or not
* Changed the ``expansions`` property of :class:`boardgamegeek.games.BoardGame`, now it returns a list of :class:`boardgamegeek.things.Thing` for each expansion the game has
* Added a property to :class:`boardgamegeek.games.BoardGame`, ``extends`` which returns a list of :class:`boardgamegeek.things.Thing` for each item this game is an extension to