A GroupBy method in Swift as seen in other languages

GroupBy.swift

extension SequenceType where Generator.Element: Comparable {
    func group() -> [[Generator.Element]] {
        return self.group { $0 == $1 }
    }
    func group(by groupBy: (Generator.Element, Generator.Element) -> Bool) -> [[Generator.Element]] {
        return self.reduce([]) { (xs, q) -> [[Generator.Element]] in
            guard xs.count > 0 else { return [[q]] }
            var xs = xs
            let last = xs.count-1
            if groupBy(xs[last][0], q) {
                xs[last].append(q)
                return xs
            }
            xs.append([q])
            return xs
        }
    }
}

["a", "a", "a", "A", "A", "b", "c", "b"].group() // [["a", "a", "a"], ["A", "A"], ["b"], ["c"], ["b"]]
["a", "a", "a", "A", "A", "b", "c", "b"].group { a, b in a.lowercaseString == b.lowercaseString } // [["a", "a", "a", "A", "A"], ["b"], ["c"], ["b"]]