Swift的操场或控制台应用程序中的以下代码：
- let letters = ["A","B","C"]
- letters.filter({
- (x : String) -> Bool in
- println("PRINT: \(x)")
- return true
- })
打印出来
- PRINT: A
- PRINT: B
- PRINT: C
- PRINT: A
- PRINT: B
- PRINT: C
为什么这两次迭代？
