(defun lefties (grammar)
  (mapcar #'car grammar))

(defun righties (grammar)
  (remove nil (flatten (mapcar #'cdr (mapcar #'cdr grammar)))))

(defun terminals (grammar)
  (remove nil (set-difference (righties grammar) (lefties grammar))))

(defun lint (grammar)
  (remove nil (set-difference (remove 'sentence (lefties grammar)) (righties grammar))))