set-exclusive-or

(set-exclusive-or list1 list2 &key test test-not key)

Returns the symmetric difference of the two lists: the elements of either list that have no match in the other. The comparison is eql by default; :test takes a function designator, :test-not matches where the given function answers false, and :key is a selector applied to both compared elements. The comparison is always made with the list1 element first, in both directions. The result lists the list1-only elements in order, then the list2-only ones (Common Lisp leaves the order unspecified).