Sequence.prefix https://github.com/apple/swift/blame/swift-3.1.1-RELEASE/stdlib/public/core/Sequence.swift#L1272 Collection.prefix https://github.com/apple/swift/blame/swift-3.1.1-RELEASE/stdlib/public/core/Collection.swift#L1408 (edited)

swift - The Swift Programming Language

swift - The Swift Programming Language