 * Make blog() handle exact cases correctly. E.g., (8/27) ** (2/3) = 4/9, so
   Math::BigRat -> new("4/9") -> blog("8/27") should return 2/3.

 * Rounding needs a new definiton: rounding to nearest fraction (see Knuth and
   fixed-slash fractions)

 * bmod(), bmodpow(), bmodinv() not fully/properly implemented yet.

 * BigRat does not handle modify() yet or in a very incomplete manner.
