import Data.Set
a = fromList ["John", "Bob", "Mary", "Serena"]
b = fromList ["Jim", "Mary", "John", "Bob"]
(-|-) :: Ord a => Set a -> Set a -> Set a
x -|- y = (x \\ y) `union` (y \\ x)
-- Equivalently: (x `union` y) \\ (x `intersect` y)