SET-DIFFERENCE function computes set difference, that is a list of elements that appear in list1 but do not appear in list2. Test argument specifies comparison operator. Default comparison operator is EQL. Key argument specifies function for extracting relevant value from list items. Default key is IDENTITY. Resulting item order is not specified. See also SET-EXCLUSIVE-OR, UNION and INTERSECTION.

(set-difference '(a b c) '(b c d)) => (A)
(set-difference '("a" "b" "c") '("b" "c" "d")) => ("c" "b" "a")
(set-difference '("a" "b" "c") '("b" "c" "d") :test #'equal) => ("a")
(set-difference '((a . 2) (b . 3) (c . 1)) '((b . 1) (c . 2) (d . 4)) :test #'equal) => ((C . 1) (B . 3) (A . 2))
(set-difference '((a . 2) (b . 3) (c . 1)) '((b . 1) (c . 2) (d . 4)) :key #'car) => ((A . 2))
(set-difference '((a . 2) (b . 3) (c . 1)) '((b . 1) (c . 2) (d . 4)) :key #'cdr) => ((B . 3))