Scala迭代器：“在调用方法之后永远不应该使用迭代器” – 为什

It is of particular importance to note that,unless stated otherwise,one should never use an iterator after calling a method on it. The two most important exceptions are also the sole abstract methods: next and hasNext.

他们还给出了安全和不安全使用的具体示例：

def f[A](it: Iterator[A]) = {
  if (it.hasNext) {            // Safe to reuse "it" after "hasNext"
    it.next                    // Safe to reuse "it" after "next"
    val remainder = it.drop(2) // it is *not* safe to use "it" again after this line!
    remainder.take(2)          // it is *not* safe to use "remainder" after this line!
  } else it
}

不幸的是,我不遵循这里不安全的想法.有人能在这里为我揭开光明吗？