Precision and CD's

[Richard J. Fateman]

I think that the rational numbers (ratios of arbitrary precision
integers) solve the problem that you seem to be addressing
(how to represent 1/7, etc.)  
Basically I think of the floats as a selection of rational
representatives from the REAL numbers, so a completion in the
rationals is not interesting.

Other considerations:

The major advantage of floats/ bigfloats  is that the cost of
operations is mostly a function of the desired precision, and 
not (usually) the value. The usual
operations include sqrt, exp, log, sin, cos  etc. as well as +-*/.

Closing the floats exactly under +  is already a bad idea because
1+1*10^1000  requires 1000 decimal digits in the fraction.

Closing the floats under division (except division by zero) just to
make them a field has some esthetic mathematical appeal, but in my
opinion it is not worth the complexity and cost of handling repeated
decimals or unknown expansion size.

RJF