adjoin

(adjoin item list &key test key)

Returns list unchanged if item is already a member; otherwise returns a new list with item prepended. The comparison is eql by default; the optional :test keyword takes a function designator to use a different comparison, and the optional :key keyword takes a selector function applied to both the item and the list elements (the prepended item stays unkeyed). It is the non-destructive way to add an element to a set without creating duplicates.