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--> s = []
--> s += 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
For comparison, when NotImplemented is appropriately returned the message looks like this:
--> s -= 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -=: 'list' and 'int'
Which is certainly more helpful than "<blank> object is not iterable"