Find images which contain one keyword but not another

I need to add a keyword to hundreds of photos which contain a certain keyword, but do not contain the keyword I want to add.  How do I search for these images to be updated.  No spaces are involved.

Note that the criterion "Keywords contains a" will find all photos having at least one keyword that contains the string "a" anywhere within a keyword. For example, "Keywords contains car" will match photos having the keyword "placard".

So as a general rule of thumb, always use the criterion "Keywords contains words a".  That will only match photos that contain "a" as a punctuation-separated "word".  E.g. "Keywords contains words car" won't match "placard".  But note that "Keywords contains words red car" would match a photo with one keyword "red flower" and another keyword "blue car".  (There are other bizarre differences between "contains" and "contains words".) 

The only way to get true exact match of keywords without using a plugin is to use the Keywords column of the Metadata browser in the Library Filter bar. That's incredibly clumsy to use when you have a large number of keywords, but there's a shortcut: In the Keyword List panel, search for the desired keyword. Then hover the mouse over the keyword and click the arrow on the right, which will open the Keywords column with that keyword selected:

Never Mind.  I finally found out how to filter on photos which include a certain keyword but not another.

Open: Filter bar

Select: Text

Select: Keywords (in this case, to search on)

I had contained as the rule, but that probably doesn't matter in this case.

Enter:  +[keyword] ![keyword]  (Assuming no spaces in the keyword, and without the brackets, of course.)