Merges lists (<l1?> and <l2?> and possibly more) into a single list with no repetitions. <test> is a function or function name for a binary comparison. <key> is a name or function name to apply to the elements before comparison. Ex. (x-union '(1 2 3 4 5) '(4 5 6 7 8)) => (8 7 6 1 2 3 4 5)