Wu-Decimal uses exact rationals (they aren't base 10 floats). Division works according to normal Lisp semantics, since the CL ratio type is used for arithmetic. Let's say you divide something by 3 and now you have infinitely repeating digits: then it is no longer in set D, and Wu-Decimal no longer considers it to be of decimal type. Instead, it is treated as a fraction, again per standard CL semantics. The "Printing" example tries to clarify this (notice that 1/2 prints as '0.5' but 1/3 remains '1/3').
I've monkeyed with a lot of numeric representations, and every one of them involves tough theoretical and practical trade-offs. Returning an exact rational as the result of division sounds reasonable.
My favorite representation so far pairs a signed float in [2^-512,2^512) with a signed integer to extend the exponent range. The idea is to avoid overflow and underflow, particularly when multiplying thousands of probabilities.
(On average, adding a few thousand log probabilities or densities and then exponentiating the sum yields a number with about 9 digits precision. Worst case for adding log probabilities and exponentiating is about 8 digits precision, and for adding log densities it's 0 digits. Multiplying the same number of probabilities or densities retains about 13 digits in the worst case.)
