You must specify a non-negative integer. 必须指定非负整数。
The resulting value must be a nonnegative integer less than the sequence's length, and the sequence is asked to assign the assigned object to its item with that index. 最后的值必须是一个小于该序列长度的非负整数，然后该序列就被请求将被赋对象赋值给它带那个指标的项。
