[Duck typing](http://en.wikipedia.org/wiki/Duck_typing): if it looks like a duck and it quacks like a duck, it's probably a duck. In this context, it means that there's no reason to verify that the variable is an instance of the class we're expecting -- just so long as it has all the same methods as a `MongoModuleStore`, that's enough. It might secretly be a `RiakModuleStore`, or even a `FakeTestingModuleStore`, and it doesn't matter: we can treat it the same way -- and if it throws an exception, well this existing code throws exceptions anyway, so we're no worse off than before.