I think I know my Lightroom pretty well, but I couldn't find a way to create a smart collection that returns photos containing only one given keyword. Just one. Say, all photos containing only the 'foo' keyword, no other keywords.
The only solution I found was:
- create a smart collection that returns photos containing that given keyword 'foo',
- select all photos and add them to another collection (say 'temp') for backup,
- remove the given keyword 'foo' from the photos,
- create a smart collection that returns the photos containing no keywords: this should return all the photos that only had the 'foo' keyword before I removed it in the previous step. Unfortunately all photos that had no keywords before step 3 will be returned as well, but that's OK, I don't have such photos.
- do something with the photos returned by step 4, then go to the 'temp' collection and add the 'foo' keyword back.