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Qualified types introduce a type predicate that needs to be checked after inferring each expression. For example, let's say the signature of '+' is 'a, a -> a' where 'a' is any type. Addition only makes sense for certain types of data, so we can then specify which types are valid with a predicate. For example: where 'a' is a float or an int. Adding two booleans is valid as far as the function signature is concerned, but it will then fail the predicate check.