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In order to create a coherent integer type class both __int__ and __index__ must be defined or the resulting instances will behave inconsistently in different places.
For example, if __index__ is not defined then the class cannot be used in slices, and if __int__ is not defined then int(integer_type) will fail.
At this point the programmer must remember to define both, but since they should return the same value we can have the type constructor automatically assign __int__ to __index__ when the latter is defined and the former is not. This would be similar to how we currently treat __ne__ when __eq__ is defined.