B  
big_int [Big_int_Z]  
K  
kind [Q] 
Rationals can be categorized into different kinds, depending mainly on
whether the numerator and/or denominator is null.

T  
t [Q] 
A rational is represented as a pair numerator/denominator, reduced to
have a nonnegative denominator and no common factor.

t [Z] 
Type of integers of arbitrary length.
