complicated but works

Took me awhile to get it working and still can’t say I understood it, but it solved the problem well! Some more explanatory text in the settings page could be helpful. Also didn’t use the terminology of posts page (which is what settings calls it) so it was hard to figure out what was excluding what. I guess this represents differences between the code and the backend so may be WordPress’s fault, but that doesn’t make it any less confusing.
Did I say it worked with my problem of excluding one category from my posts page.