[mcclim-devel] Subclassing text-field?

I would like to have a numeric-only text-field. I want it to return numbers with gadget-value, and to be able to setf it with numbers (and not strings). Is subclassing text-field (and text-field-pane?) and defining gadget-value and (setf gadget-value) the solution? If so, it seems to be more complicated than I thought. Any idea/help/example? Thanks, Yoel