code: def union(self, *sets):
          
          cls = self.__class__ if isinstance(self, OrderedSet) else OrderedSet
          containers = map(list, it.chain([self], sets))
          items = it.chain.from_iterable(containers)
          return cls(items)
docs: Combines all unique items.
          Each items order is defined by its first appearance.
  
          Example:
              >>> oset = OrderedSet.union(OrderedSet([3, 1, 4, 1, 5]), [1, 3], [2, 0])
              >>> print(oset)
              OrderedSet([3, 1, 4, 5, 2, 0])
              >>> oset.union([8, 9])
              OrderedSet([3, 1, 4, 5, 2, 0, 8, 9])
              >>> oset | {10}
              OrderedSet([3, 1, 4, 5, 2, 0, 10])