In TADS 3, a mix-in class is any class that cannot be used by itself, but must be used in conjunction with another class. Examples include Lockable, BagOfHolding, and LockableWithKey. Contrast this with a standalone class which can be used by itself.
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TODO: Questions to be answered: How does one tell if a class is a mix-in class or not? Are there not mix-in classes in TADS 2 as well? If so, what are some examples of those? Does it matter which order classes are declared when using a mix-in class with another? Perhaps see a couple examples?