SequenceType in Swift: Even numbers, Powers of two, Fibonacci and Prime numbers

Sequences.swift

//Sequence of positive even numbers 
struct Even: SequenceType {
  func generate() -> AnyGenerator<Int> {
    var _first = 0
    return AnyGenerator<Int> {
      _first += 2
      return _first
    }
  }
}

//Sequence of powers of two 
struct PowersOfTwo: SequenceType {
  func generate() -> AnyGenerator<Int> {
    var _item: Int = 0
    return AnyGenerator<Int> {
      _item = _item == 0 ? 1 : _item
      _item = _item << 1
      return _item
    }
  }
}

//Sequence of prime numbers
struct PrimeNumbers: SequenceType {
  func generate() -> AnyGenerator<Int> {
    var _pastPrimes: [Int] = []
    var _current: Int = 2
    return AnyGenerator<Int> {
      if _current == 2 {
        _current += 1
        return 2
      }
      var found: Bool = false
      var _result: Int = 2
      while !found {

        guard !(_current % 2 == 0) else {
          _current += 1
          continue
        }

        let hits = _pastPrimes.filter {_current % $0 == 0}

        guard hits.count == 0 else {
          _current += 1
          continue
        }

        _pastPrimes.append(_current)
        _result = _current
        found = true
        _current += 1

      }
      return _result
    }
  }
}

//Sequence of Fibonacci numbers

struct Fibonacci: SequenceType {
  func generate() -> AnyGenerator<Int> {
    var _last: Int = 1
    var _laster: Int = 1
    var _count: Int = 0
    var _result: Int = 0
    return AnyGenerator {
      _count += 1
      guard _count != 1 else {
        return _laster
      }
      guard _count != 2 else {
        return _last
      }
      _result = _last + _laster
      _laster = _last
      _last = _result
      return _result
    }
  }
}

var even = Even().generate()
for _ in 0..<200 {
  if let val = even.next(){
    print(val)
  }
}

var power = PowersOfTwo().generate()
for _ in 0..<10 {
  if let val = power.next(){
    print(val)
  }
}

var fibo = Fibonacci().generate()
for _ in 0..<50 {
  if let val = fibo.next(){
    print(val)
  }
}

var prime = PrimeNumbers().generate()
for _ in 0..<200 {
  if let val = prime.next(){
    print(val)
  }
}