So, here my change which should perform the wished feature:
;; the generic function to specialize on (defmethod handle-output (key params))
(defun process-one-event (event) (when event (when *debug-tk* (format *trace-output* "l:~s<=~%" event) (finish-output *trace-output*)) (cond ((and (not (listp event)) *trace-tk*) (princ event *trace-output*) (finish-output *trace-output*)) ((not (listp event)) nil) ((eq (first event) :callback) (let ((params (rest event))) (callback (first params) (rest params)))) ((eq (first event) :event) (let* ((params (rest event)) (callback (first params)) (evp (rest params)) (event (construct-tk-event evp))) (callback callback (list event)))) (t (handle-output (first event) (rest event))))))