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on this note, we should/could add a REMOVE-FROM-ALIST and their DELETE-FROM... counterparts.

I agree.

i've added the missing delete-from-plist and their modify macro variants. i'm not sure about the alist versions, though: what should be their api? plists are usually used with symbols as keys, so using #'eq for comparison is ok, but alists are mostly addressed by non-symbols, so the (alist &rest keys) api is not too useful without a :test argument. because of this i've skipped them for now.

...

i personally would be ok if they worked so that when called with NIL they would always return T (so NIL would be taken as an empty sequence). and if we added FIRST-ELEMENT/LAST-ELEMENT then people could use them to test if a sequence start and/or ends with NIL as an element.

Would STARTS-WITH-SUBSEQ and ENDS-WITH-SUBSEQ work for you?

yep, i've added them. everybody, feel free to excercise your veto rights even with an UNDO patch, i won't take it as an offense! this is how the Alexandria project is planned to work...