Nonetheless, they were fooled by that function, because it's just keep-if by a funny name that includes "not" suggesting that it contains a complement that migh

Nonetheless, they were fooled by that function, because it's just keep-if by a funny name that includes "not" suggesting that it contains a complement that might be factored out. If you want keep-if, you don't want to use a different function, which forces you to complement your predicate. If you want (keep-if #'redp jellybean-list) to keep the red jelly beans, you don't want to write (remove-if (complement #'not-red-p) jellybean-list). If keep-if has a silly name remoe-if-not, you might nonetheless prefer (remove-if-not #'redp jelly-bean-list). Shims like complements are ugly, and compilers won't optimize through them for arbitrary function definitions (whose source code is not even in scope), so it is good to have both keepers and removers. Heck, it's useful to have a function which does both in one pass returning two values: the filtrate and the retentate.