Stable sorting for swift sequences

Sequence+StableSorted.swift

extension Sequence {

    /// return a sorted collection using a stable sort algorithm
    ///
    /// - Parameter areInIncreasingOrder: return nil when two element are equal
    /// - Returns: the sorted collection
    ///
    /// Shamelessly copied from https://stackoverflow.com/a/45585365
    ///
    public func stableSorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element] {
        return try enumerated()
            .sorted(by: { lhs, rhs -> Bool in
                try areInIncreasingOrder(lhs.element, rhs.element)
                    || (lhs.offset < rhs.offset && !areInIncreasingOrder(rhs.element, lhs.element))
            })
            .map { $0.element }
    }
}