Kenny, all: I have still the Listbox problem of not getting a new-value from the listbox after selecting an item. I was able to track it down to the following function in file widget.lisp: (defmethod tk-output-selection (self new-value old-value old-value- boundp) (declare (ignorable old-value old-value-boundp)) (trc "widget.lisp: selection output" self old-value new-value) (when new-value (with-integrity (:client `(:variable ,self)) (let ((v$ (if (stringp new-value) ;; just going slow on switching over to C API before changing tk-send-value new-value (tk-send-value new-value)))) (tcl-set-var *tki* (tk-variable self) v$ (var-flags :tcl- namespace-only)))))) I do get the following line as debug output: 0> 81 widget.lisp: selection output pm-list nil 25681408 So, yeah, there it is: the value of pm-list is changing from NIL to a cffi pointer address. My cell rule (c? (value (fm-other :pm-list))) does not fire ... How should I enable some more debugging ? Thanks! Frank