* There was no real default value for this parameter (neither "" nor None would work the same as
not specifying that parameter). Now, 'None' is considered as default value, and you get
exact same behavior with this value and if not specifying it. This is important at least for
consistency, and potentially too in some esoteric cases (like generated code or so).
* Add a warning about the fact that 'default' parameter shall not be psecified when items
are given a callback function.
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