logical operators(and, or, nor, ...) for searching photos by combining tags in the "find bar"

At the moment, when searching for photos by multiple tags, the searches 
of the single tags are combined by "or". This works all right in the 
"include" case, but in the "exclude" case I guess it should be "and" by 
default.
Meaning that if I look for a photo of an animal that is neither cat nor 
dog, intuitively I would place the the tag "dog" and "cat" in the find 
bar and set them both to exclude. But now this just gives my all my 
photos, except of those showing a cat and a dog together. in other 
words: my holiday in Spain and the photos of last Christmas!
So the >> "not cat" or "not dog" << should be changed to >> "not cat" 
and "not dog" <<.
Right now I can´t think of a case where the "multiple exclude" search, 
as it is, would give a desirable result, but i guess there might be 
some, so maybe you should be able to chose.
Am I at least kind of right??
Thanks in advance
Daniel