Sleep for some number of seconds
The call (sleep n) returns nil. However, it takes
approximately n seconds of real time for this call to return.
The guard for sleep requires its argument to be a non-negative
Sleep is a Common Lisp function. See any Common Lisp documentation
for more information.
(defun sleep (n)
(declare (xargs :guard (and (rationalp n) (<= 0 n))))
(declare (ignore n))