ANSI Common Lisp 12 Numbers 12.1 Number Concepts 12.1.4 Floating-point Computations
220.127.116.11 Rule of Float and Rational Contagion
When rationals and floats are combined by a numerical function,
the rational is first converted to a float of the same format.
For functions such as + that take more than two arguments,
it is permitted that part of the operation be carried out exactly using
rationals and the rest be done using floating-point arithmetic.
When rationals and floats are compared by a numerical function,
the function rational is effectively called to convert the float
to a rational and then an exact
comparison is performed. In the case of complex numbers,
the real and imaginary parts are effectively handled individually.
18.104.22.168.1 Examples of Rule of Float and Rational Contagion