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  • Karsten Gresch thinks this is interesting:

let numberOfVowels = viewController.numberOfVowels(in: string)

From

Cover of Test-Driven iOS Development with Swift 3

Note

Says: "FirstDemoTests.swift:31:26: Value of type 'ViewController' has no member 'numberOfVowels'" => You need to build the test first. I miss a hint here (cf. Ray Wenderlich).