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* Improved error message on unexpected arguments in a call
* Distinguished between optional and mandatory arguments in conditions.
Default argument values in conditions are accepted instead of raising a misleading "missing argument" exception.
* Added a boolyness check to detect if the condition evaluation can be negated.
If the condition evaluation lacks boolyness, a more informative exception is now raised.
For example, this is important for all the code operating with numpy arrays where boolyness is not given.
* Added contract location to ``require``, ``ensure`` and ``snapshot``.
This feature had been erroneously omitted in 2.0.4.