IEnumerator vs Iterator
IEnumerator
不考虑Reset方法和原始列表的可变性，NoveNext和Current的只读向前用法：
Initially, the enumerator is positioned before the first element in the collection. You must call the MoveNext method to advance the enumerator to the first element of the collection before reading the value of Current; otherwise, Current is undefined.
Current returns the same object until MoveNext is called. MoveNext sets Current to the next element.
If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined.
输入数据，并初始化枚举器：
let ls = [0;1;2]
let enumerator = (Seq.ofList ls).GetEnumerator()
let ls = []
此时调用Current会抛出异常，印证第一段描述：
enumerator.Current
//InvalidOperationException : 枚举尚未开始。请调用 MoveNext。
我们改正代码，连续运行，直到序列的末尾：
let ls = [0;1;2]
let enumerator = (Seq.ofList ls).GetEnumerator()
enumerator.MoveNext() // true
enumerator.Current // 0
enumerator.MoveNext() // true
enumerator.Current // 1
enumerator.MoveNext() // true
enumerator.Current // 2
当再次调用MoveNext()返回false，接上段代码：
enumerator.MoveNext() // false
当MoveNext()返回false之后，如果此时调用Current会抛出异常：
enumerator.Current
// InvalidOperationException : 枚举已完成。
当MoveNext()返回false之后，继续调用MoveNext()不会抛出异常，它总是返回false：
enumerator.MoveNext() // false
enumerator.MoveNext() // false
enumerator.MoveNext() // false
enumerator.MoveNext() // false
当输入序列为空时，行为如下：
let ls = []
let enumerator = (Seq.ofList ls).GetEnumerator()
enumerator.MoveNext() // false
enumerator.Current
// InvalidOperationException : 枚举已完成。
Iterator
javaScript的枚举器用法如下：
const iterable = ['a', 'b'];
const iterator = iterable[Symbol.iterator]();
iterator.next() //, { value: 'a', done: false }
iterator.next() //, { value: 'b', done: false }
iterator.next() //, { value: undefined, done: true }
