;; TODO: air resistance, terminal velocity
(defconstant +G+ 9.81)
(defvar *alt* 20000)
(defvar *v* 0)
(defun one-step ()
  (setq *alt* (- *alt* *v*))
  (setq *v* (+ *v* +G+))
  (format *standard-output* "~A~%" *alt*))
(defun my-main ()
  (loop while (> *alt* 0)
        do (one-step)))

;;; And here's an example from GNAT:
(defun free-fall ()
  (format *standard-output* "Gravitational constant: ~A~%" +G+)
  (format *standard-output* "Twice that: ~A~%" (* 2 +G+))
  (let* ((time 10)
         (distance (* 0.5 +g+ (expt time 2))))
    (format *standard-output* "Distance travelled in 10 seconds of free fall: ~A~%" distance)))