The larger of two numbers
(Max x y) is the larger of the numbers x and y.
The guard for max requires its arguments to be rational (real, in ACL2(r)) numbers.
Max is a Common Lisp function. See any Common Lisp documentation for
(defun max (x y)
(declare (xargs :guard (and (real/rationalp x)
(if (> x y) x y))