Thank you Hassan for your kind reply:
1. I use wordpress for several sites since WP 1.5 and I was unaware of the screen option: it's great, and I feel sooo dumb!
2. I will test the functions.php approach.
3. yesterday I tried the CAC plugin, it's ok in most of my sites and has other interesting features.
PS: for reasons that may only originate from my own, I could not find a prompt reference to this issue.
Thank you again :-)