Problem 3 (20 points) Write a function, called union: 00a list ? 00a list ? 00a list, that when called with two lists, denoting sets, returns their union. In other words, union(s1,s2) = s1 ? s2 where s1 and s2 are lists. Recall that sets may not contain duplicate elements. (Hence s1 ? s2 may not contain duplicate elements).