transpose.Swift

extension SequenceType where Generator.Element: SequenceType {
  func transposeAndPad() -> [[Generator.Element.Generator.Element]]? {
    var g = self.generate()
    return g.next().map {
      var gens = GeneratorSequence(g).map{$0.generate()}
      return $0.map { fEl in [fEl] + gens.indices.map{gens[$0].next() ?? fEl}}
    }
  }
}