union

>>-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.