restriction on sum: intentional bug?

[Aahz]

In article <0062f568$0$26941$c3e8da3 at news.astraweb.com>,
Steven D'Aprano  <steve at REMOVE-THIS-cybersource.com.au> wrote:
>
>Mathematically, sum() is defined as the repeated application of the + 
>operator. In Python, the + operator is well-defined for strings and lists 
>as well as numbers. Since you can say "ab" + "cd" + "ef" and get a 
>sensible result, then sum() should be able to do the same thing.
>
>And indeed, if you pass a list-of-lists to sum(), it does:
>
>>>> sum([[1,2], ['a',None], [1,'b']], [])
>[1, 2, 'a', None, 1, 'b']
>
>(For the record, summing lists is O(N**2), and unlike strings, there's no 
>optimization in CPython to avoid the slow behaviour.)

Are you sure?
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