This will do the job, I believe -thanks.

And you are very right about (too) many options... speaking for me, it's hard to remember all those combinations. Actually, I've started writing my own "how to" notices from this forum.

Now, I'll use oportunity for another question: How to tell, if some keyword allready exist inside xmp:subject?
For example, there's allready "stiri" keyword there. Now I wish to add "tir" keyword, but only if "tir" is not already there (notice, "tir" occur inside "stiri").
I've tried with various combinations of (from this forum):
exiftool -xmp:subject+=tir -if "not $xmp:subject=~/tir/" photo.jpg
exiftool -xmp:subject+=tir -if (not $xmp:subject eq "tir") photo.jpg
...etc. -without success.

What I need is exact match. I don't need to tell you, I don't know what exactly "tag=~/xxx/" does. I saw somewhere in this forum, something like "tag=!/..." was used. Where can I find more examples of using "-if"?

Btw. In Windows, how do I use (nested) guotes for:
exiftool -ApertureValue -if '$Make eq "Canon"' *.jpg

Now, I'll shut up :-)

Bogdan