Rule of Float and Rational Contagion (ANSI and GNU Common Lisp Document)

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#### 12.1.4.1 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.