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So the new branch looks great---thanks, Stefan! I'm only just beginning to look at the code properly, though.
A couple of things:
(1) Could we unify test_decimal and test_cdecimal somehow? This would avoid them getting out of sync when new tests are added, and would make it clear what the differences between them are. It looks like there's currently a lot of duplicate code.
(2) At some point we'll need some documentation. Even if all it says is: the cdecimal module operates identically to the decimal module, with the following exceptions... (notes on threading differences, exponent limits, correct rounding of pow, etc.)