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