>>-union(argument)---------------------------------------------><

Returns a new collection of the same class as the receiver that 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 equivalent item in the receiver collection and the method has not already included. The order in which this method selects items in argument is unspecified (the program should not rely on any order). The argument can be any collection class object.