First, I’m not complaining, I just would like to understand. 
And I ask because I did not find it in the manual.
This search finds all documents with “cat” but without dog. This works.
cat -dog
This search should find all documents with “cat” but without “dog” and without “mouse”. This does not work.
cat -dog -mouse
Does that mean that you can exclude only one string/word? Thank you.