(defun nextState (st act)
  "generate the nextState after state st and action act from prolem"
  (let ((new-state (make-state :action act :track (state-track st))))
    (setf (state-vel new-state)
      (make-vel (+ (vel-l (state-vel st)) (acce-l act))
            (+ (vel-c (state-vel st)) (acce-c act))))
    (setf (state-pos new-state)
      (make-pos (+ (pos-l (state-pos st)) (vel-l (state-vel new-state)))
            (+ (pos-c (state-pos st)) (vel-c (state-vel new-state)))))
    (setf (state-cost new-state)
      (cond ((isGoalp new-state) -100)
        ((isObstaclep (state-pos new-state) (state-track new-state)) 20)
        (T 1)))
    (when (= (state-cost new-state) 20)
      (setf (state-vel new-state) (make-vel 0 0))
      (setf (state-pos new-state) (make-pos (pos-l (state-pos st))
                        (pos-c (state-pos st)))))
    (values new-state)))