While this is a very good thing to do, as it protects you quite well, it can cause further problems when it comes to user expectations. You have to make sure that the contract between this object and the rest of the world is well-documented. Otherwise, a user will pass in 'bird' and not know why 'bird' isn't there.

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.