the absolute value of a real number
Major Section:  PROGRAMMING

(Abs x) is -x if x is negative and is x otherwise.

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

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

From ``Common Lisp the Language'' page 205, we must not allow complex x as an argument to abs in ACL2, because if we did we would have to return a number that might be a floating point number and hence not an ACL2 object.