You must specify a non-negative integer. 必須指定非負整數。
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. 最后的值必須是一個小于該序列長度的非負整數,然后該序列就被請求將被賦對象賦值給它帶那個指標的項。