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> In the C version of decimal, do distinct Decimal objects ever share 
> coefficients? (This would be an obvious optimization for methods 
> like Decimal.copy_negate; I don't know whether the C version 
> applies such optimizations.) If there's potential for shared 
> coefficients, that might make the "not count any memory twice" part 
> tricky.
I know of three strategies to deal with such a case:
a) expose the inner objects, preferably through tp_traverse, and
 don't account for them in the container,
b) find a "canonical" owner of the contained objects, and only
 account for them along with the canonical container.
c) compute the number N of shared owners, and divide the object
 size by N. Due to rounding, this may be somewhat incorrect.