(defun show-class-tree (class-designator &optional (level 0))
  (let ((class (if (typep class-designator 'class)
                   class-designator (find-class class-designator))))
    (format t "~vt~(~a~)~%" level (class-name class))
    (loop for c in (mop:class-direct-subclasses class)
	  :do (show-class-tree c (1+ level)))))