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iOS Generics in Swift Generic Functions, Parameters and Constraints CSS Background Blend Modes Challenge

Type [T] to type [U]

I don't understand the challenge, it's suppose to take an array of type T, and then with a function that takes a T for a parameter and returns a U, you should modify the [T] that the map function takes, but how am I suppose to return it later if the return is [U] type. Am I missing something? BTW I can't find the square function nor anything like that. Please help :(