Method: (BACKTRACK-SLOT STANDARD-SESSION-FRAME T T)

Source

(defmethod backtrack-slot ((frame standard-session-frame) object slot-name &optional (copyer #'identity))
  (backtrack frame
             (make-instance 'place
                            :getter (lambda ()
                                      (if (slot-boundp object slot-name)
                                          (slot-value object slot-name)
                                          +unbound-value+))
                            :setter (lambda (v)
                                      (if (eql +unbound-value+ v)
                                          (slot-makunbound object slot-name)
                                          (setf (slot-value object slot-name) v)))
                            :copyer copyer
                            :form `(slot-value ,object ,slot-name))))