(Programming sets as lists in Dr. Racket)

Suppose that sets are implemented as lists, where each element of a set appears exactly once in its list. Example: Set {a, b, c} corresponds to list S = (a b c). Define, execute, and thoroughly test a recursive LISP (Dr. Racket) program that:

Constructs the union of two sets, i.e., (union S1 S2) returns a list that equals S1 S2. (Warning: If you implement this by simply appending the two lists, the same element can incorrectly appear more than once in the list for the union.)

Please provide the code you used as well as a screen shot of it executing