Find non-repetitive combinations of a set

extension Set {
  private func addCombination(prevCombo: [Element], pivotList: [Element]) -> [([Element], [Element])] {
    var pivotList = pivotList
    return (0..<pivotList.count)
      .map { _ -> ([Element], [Element]) in
        (prevCombo + [pivotList.remove(at: 0)], pivotList)
    }
  }

  func combinations(m: Int) -> [[Element]] {
    var initial = [([Element](), Array(self))]
    for _ in 1...m {
      initial = initial.flatMap(addCombination)
    }
    return initial.map{$0.0}
  }
}

let set = Set([1,2,3,4])
set.combinations(m: 3)
// [[3, 1, 4], [3, 1, 2], [3, 4, 2], [1, 4, 2]]