as undone.

   Finally, if you neither insert nor delete text, but directly modify
the existing text (e.g., change its case), call 'rl_modifying()' once,
just before you modify the text.  You must supply the indices of the
text range that you are going to modify.

 -- Function: int rl_modifying (int start, int end)
     Tell Readline to save the text between START and END as a single
     undo unit.  It is assumed that you will subsequently modify that
     text.