RecursiveSelectGenFromGen.Swift

public class SelectGenerator<T> : GeneratorType {
  
  private var head: T?
  private var tail: [T] = []
  private var first: Bool = true
  private var gen: SelectGenerator<T>?
  
  public func next() -> (T, [T])? {
    return first ? {
      first = false
      gen = SelectGenerator(g: tail.generate())
      return (head!, tail)
    }() : gen?.next().map { ($0, [head!] + $1) }
  }
  
  private init<G : GeneratorType where G.Element == T>(var g: G) {
    if let h = g.next() {
      self.head = h
      self.tail = Array(GeneratorSequence(g))
    } else {
      self.first = false
    }
  }
}

public func select<G : GeneratorType>(gen: G) -> SelectGenerator<G.Element> {
  return SelectGenerator(g: gen)
}

var g = select([1, 2, 3, 4].generate())

g.next() // (1, [2, 3, 4])
g.next() // (2, [1, 3, 4])
g.next() // (3, [1, 2, 4])
g.next() // (4, [1, 2, 3])
g.next() // nil

var uG = [1].generate()

uG.next() // uG is now empty.

var eG = select(uG) // previously would have crashed, with a force-unwrap. Now safe.

eG.next() // nil