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I don't see anything objectionable about the class optimizing the implementation of a private method.
I'll note that there's a speed benefit beyond just removing the two type checks in the common case: the optimized `_randbelow()` also avoids populating its locals with 5 unused formal arguments (which are just "a trick" to change what would otherwise have been global accesses into local accesses). So it actually makes the method implementation cleaner & clearer too.
But it's really the speed that matters here.