See Paige's response below.
Are you trying to filter the Keyword list, or filter images using keywords?
What you're asking can't be done to filter the Keywords list, but you can create a Smart Collection to filter images by keyword using multiple rules like "find images that contain keyword A but not keyword B."
This is interesting. I want to filter photos using multiple keywords. Essentially I would like to filter using Boolean logic. Show photos of Eleanor in San Francisco after 2010 but not with Elayne in them.
Can I search using multiple lines and for A but not B?
In the text filter, type in A !B
The exclamation point indicates NOT B
More copmlicated boolean searches can usually be performed using a Smart Collection.
Learn something new every day!
Is this documented somewhere?
Yes, I have seen it in the Lightroom help files