indicator for positive, negative, or zero
Major Section:  PROGRAMMING

(Signum x) is 0 if x is 0, -1 if x is negative, and is 1 otherwise.

The guard for signum requires its argument to be rational (real, in ACL2(r)) number.

Signum is a Common Lisp function. See any Common Lisp documentation for more information.

From ``Common Lisp the Language'' page 206, we see a definition of signum in terms of abs. As explained elsewhere (see abs), the guard for abs requires its argument to be a rational (real, in ACL2(r)) number; hence, we make the same restriction for signum.