This is in SML.

Problem 3 (20 points) Write a function, called union, "a list * "a list ? "a list, that when called with two lists, denoting sets, returns their union. In other words, union(s1,s2) s1 Us2 where s1 and s2 are lists Recall that sets may not contain duplicate elements. (Hence s1 Us2 may not contain duplicate elements)