Returns a new Set contains all the items from the receiver collection and selected items from the argument collection. This method includes an item from argument in the new collection only if there is no item already in the in the receiver collection and the method has not already included a matching item. The order in which this method selects items in argument is unspecified. The argument can be a Collection object or any other object that supports a makeArray method. The argument must also allow all of the index values in the receiver collection.