## ABS

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.