(defun guess (min max)
  (let ((guess (round (/ (+ min max) 2))))
    (format t "My guess: ~a. Is your number (l)ower, (h)igher or (e)qual to this? " guess)
    (let ((answer (read-line)))
      (cond ((equalp answer "l") (guess min (1- guess)))
            ((equalp answer "h") (guess (1+ guess) max))
            ((equalp answer "e") (format t "Yeah!~%"))
            (t (guess min max))))))

(guess 0 100)