In the manual, there are zle -T functions mentioned:

              -T     This is used to add, list or remove internal
 transforma‐
                     tions on the processing performed by the line editor.
It
                     is typically used only for debugging or  testing  and
 is
                     therefore of little interest to the general user.

Could someone provide an example of such transformation function?

-- 
Best regards,
Sebastian Gniazdowski