ANSI Common Lisp 12 Numbers 12.1 Number Concepts 12.1.3 Rational Computations
188.8.131.52 Rule of Canonical Representation for Rationals
If any computation produces a result that is a mathematical ratio of two integers
such that the denominator evenly divides the numerator, then the result is converted
to the equivalent integer.
If the denominator does not evenly divide the numerator,
the canonical representation of a rational number is as the ratio
that numerator and that denominator, where the greatest common divisor of
the numerator and denominator is one, and where the denominator is positive
and greater than one.
When used as input (in the default syntax),
the notation -0 always denotes the integer 0.
A conforming implementation must not have a
representation of "minus zero" for integers
that is distinct from its representation of zero for integers.
However, such a distinction is possible for floats;
see the type float.