SmallestUsingTest.swift

extension Sequence {

    func smallest(_ n: Int, usingTest areInIncreasingOrder: (Element, Element) -> Bool) -> [Element] {
        var result = self.prefix(n).sorted(by: areInIncreasingOrder)

        for e in self.dropFirst(n) {
            if let insertionIndex = result.firstIndex(where: { areInIncreasingOrder(e, $0) }) {
                result.insert(e, at: insertionIndex)
                result.removeLast()
            }
        }

        return result
    }

}