Union and Set Difference
The union (written ∪) of two sets is the set of elements that are members of either set.
(union '(a b c) '(a c e)) -> (b a c e)
The set difference (written -) of two sets is the set of elements that are members of the first set but not the second set.
(set-difference '(a b c) '(a c e)) -> (b)Note that set difference is asymmetric: unique members of the second set, such as e above, do not appear in the output.