TreeTraversal.swift

// You could just order the elements into an array, and then return that array's generator

extension Tree {
  func inOrder() -> [Element] {
    switch self {
    case let .Node(_,l,e,r): return l.inOrder() + [e] + r.inOrder()
    case .Empty: return []
    }
  }
}

// You can still do it lazily with generators, though. 

extension Tree : SequenceType {
  func generate() -> AnyGenerator<Element> {
    switch self {
    case .Empty: return anyGenerator { nil }
    case let .Node(_, l, e, r):
      var (g, crossed) = (l.generate(), false)
      return anyGenerator {
        if let next = g.next() { return next }
        if crossed { return nil }
        (g, crossed) = (r.generate(), true)
        return e
      }
    }
  }
}